NATO Secretary-General Anders Fogh Rasmussen said Thatcher "was an extraordinary politician who was a staunch defender of freedom, a powerful advocate of NATO and the transatlantic bond."
Israeli Prime Minister Benjamin Netanyahu said Thatcher had inspired a generation of political leaders.
"She was truly a great leader, a woman of principle, of determination, of conviction, of strength; a woman of greatness," he said. "She was a staunch friend of Israel and the Jewish people."
Canadian Prime Minister Stephen Harper said the world had lost a "giant among leaders" with Thatcher's passing.
She "had that rarest of abilities to herself personify and define the age in which she served. Indeed, with the success of her economic policies, she defined contemporary conservatism itself," he said.
Nigeria's President Goodluck Jonathan said she would be remembered "for her very unique, distinctive and purposeful leadership which restored pride and respect to her country" and put it back on the global stage.
'Love her or hate her'
Hollywood star Meryl Streep's portrayal of Thatcher in the 2011 film "The Iron Lady" gave some younger viewers their first insight into the former politician's character.
"To me she was a figure of awe for her personal strength and grit," Streep said Monday.
"To have withstood the special hatred and ridicule, unprecedented in my opinion, leveled in our time at a public figure who was not a mass murderer; and to have managed to keep her convictions attached to fervent ideals and ideas -- wrongheaded or misguided as we might see them now -- without corruption -- I see that as evidence of some kind of greatness, worthy for the argument of history to settle."
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.
And 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 the nation'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."
The current speaker of the UK House of Commons, John Bercow, said he was greatly saddened by her death.
As a 16-year-old student in Thatcher's own constituency at the time of the 1979 general election which put her in power, he is part of a generation of British politicians to have grown up in her shadow.
Parliament will come back early from recess to allow lawmakers to pay their tributes to Thatcher at a special session on Wednesday, Cameron said.
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."