David Cameron, leader of the Conservative Party:
"Today is a truly sad day for our country. We've lost a great Prime Minister, a great leader, a great Briton. As our first woman Prime Minister Margaret Thatcher succeeded against all the odds, and the real thing about Margaret Thatcher is that she didn't just lead our country, she saved our country. I believe she'll go down as the greatest British peacetime Prime Minister.
"Today most of all we should think of her family. We've lost somebody great in our public life but they've lost a much loved mother and grandmother and we should think of them today. Her legacy will be the fact that she served her country so well and that she saved our country, and that she showed immense courage in doing so, and people will be learning about what she did and her achievements in decades, probably centuries to come. That's her legacy, but today we must also think of her family."
Click here to read David Cameron's full tribute.
John Major, former Conservative British Prime Minister:
"In government, the UK was turned around under - and in large measure because of - her leadership.
"Her reforms of the economy, trades union law, and her recovery of the Falkland Islands elevated her above normal politics, and may not have been achieved under any other leader.
"Her outstanding characteristics will always be remembered by those who worked closely with her: courage and determination in politics, and humanity and generosity of spirit in private."
Barack Obama, US President:
"With the passing of Baroness Margaret Thatcher, the world has lost one of the great champions of freedom and liberty, and America has lost a true friend.
"As a grocer's daughter who rose to become Britain's first female prime minister, she stands as an example to our daughters that there is no glass ceiling that can't be shattered.
"As prime minister, she helped restore the confidence and pride that has always been the hallmark of Britain at its best.
"And as an unapologetic supporter of our transatlantic alliance, she knew that with strength and resolve we could win the Cold War and extend freedom's promise.
"Here in America, many of us will never forget her standing shoulder to shoulder with President Reagan, reminding the world that we are not simply carried along by the currents of history-we can shape them with moral conviction, unyielding courage and iron will.
"Michelle and I send our thoughts to the Thatcher family and all the British people as we carry on the work to which she dedicated her life-free peoples standing together, determined to write our own destiny."
Angela Merkel, German Chancellor:
"She was an extraordinary leader in the global politics of her time.
"I will never forget her part in surmounting the division of Europe and at the end of the Cold War.
"As she took the highest democratic offices as a woman before that was common, she set an example for many.
"The liberty of the individual stood at the heart of her convictions - Margaret Thatcher recognised the power of the freedom movements of Eastern Europe early on and lent them her support."
Tony Blair, former Labour British Prime Minister:
"Very few leaders get to change not only the political landscape of their country but of the world. Margaret was such a leader. Her global impact was vast. And some of the changes she made in Britain were, in certain respects at least, retained by the 1997 Labour Government, and came to be implemented by governments around the world."
George Osborne, Chancellor of the Exchequer:
"Margaret Thatcher's belief in freedom & optimism about the future overcame all. Her determination is our generation's inspiration."
William Hague, British Foreign Secretary and MP for Richmond:
"She changed our country forever and all of us owe so much to her. A legacy few will ever equal. Rest in peace Margaret."
Boris Johnson, Mayor of London:
"Britain has lost its greatest Prime Minister since Winston Churchill.
"Margaret Thatcher freed millions of people to buy their own homes and buy shares in British companies. She ended the defeatism and pessimism of the post-war period and unleashed a spirit of enterprise.
"She fought against the clubby, cosy, male-dominated consensus of both main parties - and she won. Her beliefs - in thrift, hard work, and proper reward for merit - were not always popular. But her legacy is colossal.
"She was right about the unions, she was right about Soviet communism and recent events have shown that she was completely right about the euro. This country is deeply in her debt. Her memory will live long after the world has forgotten the grey suits of today's politics."
Ban Ki-moon, United Nations Secretary-General:
"At this time, I'd like to tell how much we will miss her great leadership. She was a pioneering leader for her contribution to peace and security, particularly at the height of the Cold War.
"She was also a great model as the first woman Prime Minister of the United Kingdom, who not only demonstrated her leadership but has given such great hope for many women for equality, gender equality in Parliament.
"We will owe a great deal to her leadership. I hope that her leadership will inspire many people around the world for peace and security and human rights.
"I hope that she will rest in peace and eternity.
"My deepest condolences and sympathy to the people of Great Britain and her family."
George W Bush, former US President:
"Laura and I are saddened by the death of Baroness Margaret Thatcher. She was an inspirational leader who stood on principle and guided her nation with confidence and clarity.
"Prime Minister Thatcher is a great example of strength and character, and a great ally who strengthened the special relationship between the United Kingdom and the United States.
"Laura and I join the people of Great Britain in remembering the life and leadership of this strong woman and friend."
George Bush Sr, former US President:
"Barbara and I were deeply saddened to learn of Baroness Thatcher's passing, and extend our heartfelt condolences to her children and loved ones.
"Margaret was, to be sure, one of the 20th Century's fiercest advocates of freedom and free markets - a leader of rare character who carried high the banner of her convictions, and whose principles in the end helped shape a better, freer world.
"The personal grief we Bushes feel is compounded by the knowledge that America has lost one of the staunchest allies we have ever known; and yet we have confidence that her sterling record of accomplishment will inspire future generations.
"May God bless the memory of Margaret Thatcher."
Condoleeza Rice, former US Secretary of State:
"I am deeply saddened by the news of the passing of Baroness Thatcher.
"There was no more potent force in the defense of freedom than this remarkable woman.
"Her unshakable belief in the universal appeal of liberty helped to steer the free world through treacherous times until communism was defeated and Europe emerged whole, free and a peace.
"I was honored to know her and to be inspired by her. Baroness Thatcher will be missed but generations will draw strength from the memory of her passion for freedom."
Mikhail Gorbachev, former President of the Soviet Union:
"The news about the death of Margaret Thatcher is a sad tiding.
"Thatcher was a politician whose word carried great weight. Our first meeting in 1984 set in train relations that were sometimes complicated, not always smooth, but which were serious and responsible on both sides. Human relations also gradually took shape, becoming more and more friendly. In the end we managed to achieve a mutual understanding, and that contributed to a change in the atmosphere between our country [the Soviet Union] and the West and the end of the Cold War.
"Margaret Thatcher was a heavyweight politician and a striking person. She will remain in our memories, and in history."
Enda Kenny, Irish Taoiseach:
"Mrs Thatcher was a formidable political leader who had a significant impact on British, European and world politics. During her 11 years as prime minister, she defined an era in British public life.
"While her period of office came at a challenging time for British-Irish relations, when the violent conflict in Northern Ireland was at its peak, Mrs Thatcher signed the Anglo-Irish Agreement which laid the foundation for improved North-South cooperation and ultimately the Good Friday Agreement.
"I extend my deepest sympathies to her family and the Prime Minister David Cameron."
Julia Gillard, Australian Prime Minister:
"Her service as the first female Prime Minister of the United Kingdom was a history-making achievement.
"Her strength of conviction was recognised by her closest supporters and her strongest opponents. I extend my sincere condolences and those of my fellow Australians to her family and friends."
Dr Manmohan Singh, Indian Prime Minister:
"I express my deepest sadness on the passing away of former British Prime Minister Baroness Margaret Thatcher."
Benjamin Netanyahu, Israeli Prime Minister:
"I send my most sincere condolences to Mrs Thatcher's family and to the government and people of Great Britain.
"Margaret Thatcher was a staunch friend of Israel and of the Jewish people.
"Mrs Thatcher was truly a great leader; A woman of principle, determination, strength & greatness."
Vaclav Klaus, former Czech President and Prime Minister:
"Thatcher was one of the greatest politicians of our time, in the Czech Republic she was our hero."
Alex Salmond, First Minister of Scotland and Leader of the Scottish National Party:
"Truly formidable prime minister whose policies defined a political generation."
John McCain, United States Senator:
"RIP Margaret Thatcher, one of the great leaders of the 20th century."
Newt Gingrich, US politician:
"Margaret Thatcher, Pope John Paul and Ronald Reagan changed history. The world would be a much different place without them."
Lord Saatchi, Chairman of the Centre for Policy Studies:
"Everyone wants to be immortal. Few are. Mrs Thatcher is. Why? Because her values are timeless, eternal. Tap anyone on the shoulder anywhere in the world, and ask what Mrs Thatcher "believed in", and they will tell you. They can give a clear answer to what she "stood for".
"She developed all the winning arguments of our time - free markets, low tax, a small state, independence, individuality, self-determination. The result was a revolution in economic policy and three election victories in a row."
Lord Bell, advisor to Lady Thatcher:
"I worked very closely with her from the moment she became leader right through until she resigned, and she was the most extraordinary woman.
"It was a great privilege to have worked for her and an even greater privilege to live in a country she ran.
"She'll probably be remembered as the greatest prime minister we've had. I don't think we'll ever see the like of her again."