Lady Thatcher, Britain's first female prime minister and among the most influential and divisive political leaders the country has ever seen, died on Monday at the age of 87 after suffering a severe stroke.

The former Conservative leader had been in ill health for more than a decade, and was staying in a suite at the Ritz hotel in London while recuperating from a minor operation.

David Cameron, who was in Madrid on a European tour, immediately returned to Britain to announce parliament was being recalled from its Easter recess, describing her as Britain's "patriot prime minister".

In a statement in Downing Street, delivered a few feet from where Thatcher quoted St Francis of Assisi as she entered No 10 for the first time in 1979, Cameron praised the "lion-hearted love" for Britain of the country's longest-serving prime minister in the 20th century.

Thatcher's friend and spokesman, Lord Bell, who was informed of her death at midday on Monday, said in a statement: "It is with great sadness that Mark and Carol Thatcher announced that their mother, Baroness Thatcher, died peacefully following a stroke this morning."

The Queen expressed her sadness and has sent a private message of condolence to the family.

Tributes flooded in from around the world for the grocer's daughter who would become one of the most divisive political figures in modern-day British politics.

Barack Obama, said the "world has lost one of the great champions of freedom and liberty, and America has lost a true friend. "She stands as an example to our daughters that there is no glass ceiling that can't be shattered."

Nancy Reagan, the widow of former US president Ronald Reagan, noted the "very special relationship" between her late husband and the former British prime minister, developed "during one of the most difficult and pivotal periods in modern history".

Reagan said: "Ronnie and Margaret were political soul mates, committed to freedom and resolved to end communism. As prime minister, Margaret had the clear vision and strong determination to stand up for her beliefs at a time when so many were afraid to 'rock the boat."

She is to be accorded the rare honour of a ceremonial funeral with full military honours at St Paul's Cathedral, central London, followed by a private cremation.

Former US president Ronald Reagan and Margaret Thatcher together at Camp David in December 1984. Photograph: Rex Features

The union flag above No 10 Downing Street, the home she inhabited for longer than any other modern British prime minister, was lowered to half-mast – as were those above Buckingham Palace and many other landmarks across the country.

The flag was flying at half-mast too at the visitor centre in Port Stanley in the Falkland Islands, the British outpost to which she dispatched a taskforce to rebuff an Argentinian invasion, sealing her reputation as the "Iron Lady".

Within an hour of her death being announced, flowers were being left outside the London home she had become too frail to live in unaided in recent months, as well as the premises of the grocer's shop in Grantham where she grew up.

In her last days, she had seemed peaceful, according to friends.

An intimate account of her last hours was given on Monday by Lady (Carla) Powell, the Italian wife of Thatcher's former diplomatic adviser Lord Powell, who had visited her often in her declining years, and whose house outside Rome the former prime minister had visited on several occasions.

Lady Powell said her husband had told her after leaving Thatcher on Sunday night that he had sat beside her bed. "She dozes off to sleep quite often and then she wakes up and we chat for a bit," she quoted her husband as saying. Lord Powell had said he had talked to her about his wife's dogs, the garden of their Italian home and the new pope. He had said he was not quite sure how much of all this Thatcher had taken in. "But she smiles a lot and seems peaceful," Lady Powell quoted him as saying.

Cameron, who cut short a visit to Spain and France to return to Downing Street after the announcement of Thatcher's death, wore a black tie as he hailed "the shopkeeper's daughter from Grantham who made it to the highest office in the land". The prime minister said: "Today we lost a great leader, a great prime minister and a great Briton. Margaret Thatcher didn't just lead our country – she saved our country." He acknowledged that Thatcher had been a divisive figure but he sought to find consensus before the special sitting of parliament on Wednesday as he acknowledged her patriotism. "We can't deny that Lady Thatcher divided opinion," Cameron said. "For many of us, she was and is an inspiration. For others she was a force to be defined against.

"But if there is one thing that cuts through all of this – one thing that runs through everything she did – it was her lion-hearted love for this country. She was the patriot prime minister and she fought for Britain's interests every single step of the way."

A Guardian/ICM poll of 965 adults online, conducted in the hours after her death, found that half of all respondents, 50%, look back on Thatcher's contribution as a positive one for Britain – 16 points more than the 34% who say she was bad for the country.

David Cameron makes statement in Downing Street on the death of Margaret Thatcher. Photograph: Ray Tang/Rex Features

Some 25% rate her record as "very good", and most of her detractors, 20% of the overall sample, deem it to have been "very bad". Only 11% sit on the fence and say she was "neither good nor bad", and just 5% told ICM that they did not know.

Two-thirds, or 62%, said her example played an important part in "changing attitudes about the role in society that women can play", whereas only half as many – 31% – believe that she changed little about gender relations in wider society "because she played by men's rules".

Thatcher's death had an immediate impact on today's political world. Ed Miliband and Nick Clegg, who had launched their campaigns for the local elections earlier in the day, announced the suspension of the campaigns.

Miliband struck a note of consensus when he said: "David Cameron, Nick Clegg and myself were all shaped by the politics of Lady Thatcher and the way she defined the politics particularly of the 1980s."

The Labour leader acknowledged that Thatcher was one of the few politicians to have succeeded in reshaping British politics. "She will be remembered as a unique figure. She reshaped the politics of a whole generation. She was Britain's first woman prime minister. She moved the centre ground of British politics and was a huge figure on the world stage."

Clegg said: "Margaret Thatcher was one of the defining figures in modern British politics. Whatever side of the political debate you stand on, no one can deny that as prime minister she left a unique and lasting imprint on the country she served. She may have divided opinion during her time in politics but everyone will be united today in acknowledging the strength of her personality and the radicalism of her politics."

But Neil Kinnock, the former Labour leader who was comprehensively defeated by Thatcher in the 1987 general election, broke with the consensus when he described her premiership as an "unmitigated disaster for Britain". In a pre-recorded BBC interview, he said: "It was an unmitigated disaster for Britain. It commenced with a series of budget changes and use of interest rates which, combined with the fact that oil was monumentally coming on stream, pushed the price of the pound out of sight and succeeded in inflicting devastating harm on the productive base in Britain. That wasn't modernisation. That was devastation."

Sir John Major, who succeeded Thatcher as prime minister in November 1990 after a challenge to her leadership, praised her as a remarkable leader. Major, who had a testy relationship with Thatcher after she encouraged the Maastricht rebels in the 1990s, told the BBC: "It is a very sad day for this country. She was a remarkable prime minister and she did some extraordinary things. In many ways she was exactly the right prime minister for the time she achieved that office."

Major praised the way in which Thatcher had managed to impose her will. "It seemed for much of the 1960s and 1970s as though governments were inevitably blown apart by the wind whatever happened. She changed that. She made the wind instead of being bent by it."But he added that the Conservative party took a long time to recover from her downfall. "Matricide is always difficult to overcome. The way in which Margaret left, rather than being defeated in a general election, left a big scar on the Conservative party. That scar continued to impact upon the party internally for a very long time."

Clare Short, Labour's former international development secretary, told the PM programme on Radio 4: "One shouldn't speak ill of the dead and she was an old lady and she was prime minister for a long time. Honour that. But from where I see it her record is very bad. In inner Birmingham unemployment went through the roof, we had riots. We never had that before. It had always been a city of full employment."

Lord Oakeshott of Seagrove Bay, the former Lib Dem treasury spokesman, said that Thatcher's reforms had ensured that the current recession has been less painful. "I always fought against Mrs Thatcher but we'd be back to three million unemployed in this recession if she hadn't made the labour market more flexible."