LONDON (CNN) -- Prime Minister David Cameron and Queen Elizabeth II led the tributes to Britain's "Iron Lady," former Prime Minister Margaret Thatcher, following the news of her death Monday.
"The Queen was sad to hear the news of the death of Baroness Thatcher. Her Majesty will be sending a private message of sympathy to the family," a statement from Buckingham Palace said.
A statement from Cameron's office said: "It was with great sadness that I learned of the death of Lady Thatcher. We have lost a great leader, a great prime minister and a great Briton."
Cameron is cutting short a trip to Spain and returning to London.
Former U.S. Secretary of State Henry Kissinger told CNN, "She was a tremendous prime minister, she was a great lady, she had very strong opinions. And to those of us who knew her over the decades she was a very warm person, which is not the public image that is often given."
Thatcher's great achievement for the country was its success in the Falklands crisis, he said, referring to the war over the disputed islands known to Argentina, which also claims them, as Las Malvinas.
"For the United States it was her staunch loyalty and commitment to the Atlantic alliance -- she was a reliable and steady ally."
She was also one of the first leaders to see the way forward to ending the Cold War, he said, spotting the potential for a new kind of Soviet leader in Mikhail Gorbachev even before he was at the helm of the Soviet Union.
Her dealings with former U.S. president and Republican icon Ronald Reagan were notably warm, Kissinger said.
"Margaret Thatcher in her relationship with Ronald Reagan gave it an additional personal dimension and it was unusually close, and they acted between themselves almost as if they were part of the same government."
There was "something very feminine and personal about her," in contrast to her public image, Kissinger added.
'Towering political figure'
Gorbachev too paid tribute to a "great politician" with a strong voice, who helped shape 20th century history.
"Our first meeting in 1984 marked the beginning of a relationship that was at times difficult, not always smooth, but on both sides serious and responsible," he said.
"Gradually, our relationship became more and more friendly. At the end, we were able to reach an understanding, and it was a contribution to the change in atmosphere between our country and the West and to the end of the Cold War."
Former British Prime Minister Tony Blair, who led the opposition Labour Party in government from 1997 to 2007, said Thatcher was a "towering political figure" who would be greatly missed.
"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," he said in a statement.
Some of the changes she made in Britain were retained to a degree by his government, he said, and their influence spread around the world.
"As a person she was kind and generous spirited and was always immensely supportive to me as Prime Minister although we came from opposite sides of politics.
"Even if you disagreed with her as I did on certain issues and occasionally strongly, you could not disrespect her character or her contribution to Britain's national life."
Bush recalls 'inspirational leader'
Former U.S. President George W. Bush said he and his wife, Laura, were grieved by the loss of a "strong woman and friend" in Thatcher.
"She was an inspirational leader who stood on principle and guided her nation with confidence and clarity," he said. "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."
European Commission President Jose Manuel Barroso said he was "deeply saddened" by the news of Thatcher's death.
"She was without doubt a great stateswoman, the first female Prime Minister of her country, and a circumspect yet engaged player in the European Union. She will be remembered for both her contributions to and her reserves about our common project," he said in a statement.
"She signed the Single European Act and helped bring about the Single Market. She was a leading player in bringing into the European family the Central and Eastern European countries which were formerly behind the Iron Curtain."
'Great hurt to Irish and British people'
But Northern Ireland politician and Sinn Fein leader Gerry Adams gave a very different view of Thatcher's legacy.
"Margaret Thatcher did great hurt to the Irish and British people during her time as British Prime Minister," he said in a statement.
"Working class communities were devastated in Britain because of her policies.
"Her role in international affairs was equally belligerent whether in support of the Chilean dictator Pinochet, her opposition to sanctions against apartheid South Africa; and her support for the Khmer Rouge.
"Here in Ireland her espousal of old draconian militaristic policies prolonged the war and caused great suffering."
She will be remembered in particular for "her shameful role during the epic hunger strikes of 1980 and 81" and her Irish policy "failed miserably," he concluded.
'Love her or hate her'
Iain Dale, author of the biography "Margaret Thatcher: In Her Own Words," told CNN the politician was unique in the way that her legacy still dominates British politics, long after she left office.
She polarized the country because she was a politician of conviction, he said.
"You see the reaction on Twitter to her death. You have people who absolutely idolized her who are devastated today, and you have others who are spewing out the most hateful things," he said.
"That's what happens when you are a conviction politician -- people either love you or hate you."
Thatcher's unbending opposition to the trade unions is behind many people's dislike, he said.
But, Dale said, she transformed the British economy, pulling it out of a malaise and restoring people's sense of self-belief.
When he last met her, about three years ago, she was still sharp when it came to remembering details of her time in office, Dale said, although her dementia affected her grasp of what was happening in the present.
Her electoral success can be credited to her ability to tap into the national psyche, he said, in the same way that Tony Blair managed in the 1990s.
"She understood what drove 'middle Britain,'" he said. "She was both a conviction politician and a popularist -- and that kind of politician is usually quite popular."
Mayor of London Boris Johnson said on Twitter: "Her memory will live long after the world has forgotten the grey suits of today's politics."
-- CNN's Roger Clark, Per Nyberg, Jonathan Wald, Richard Allen Greene and Laura Perez Maestro, and journalist Peter Taggart contributed to this report.
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