Police were braced on Saturday for a mass "party" staged by opponents of Margaret Thatcher in London's Trafalgar Square to celebrate her death.
Mayor of London Boris Johnson said the authorities were prepared for potential violence, after trouble erupted at several impromptu street celebrations following the death of the former British prime minister on Monday.
Any unrest is likely to feed concerns about security at the Iron Lady's funeral in the capital on Wednesday, which will be attended by Queen Elizabeth II, Prime Minister David Cameron and 2,000 guests, including many political and world figures.
No formal demonstration has been organised for Saturday, but over 1,500 people have pledged on Facebook to attend a "Thatcher's Dead" party at Trafalgar Square at 6:00 pm (1700 GMT).
Representatives of the coal miners who battled Thatcher in a year-long strike in 1984-5, before accepting sweeping pit closures, were planning to travel to the capital for the event alongside students and left-wing groups, The Guardian reported.
Some football fans were threatening to confront the anti-Thatcher revellers on their way home from the FA Cup semi-final between Millwall and Wigan in Wembley, north-west London, The Daily Telegraph reported.
Trafalgar Square is one of London's biggest tourist hubs, home to the Nelson's Column monument and the National Gallery.
A spokesman for Scotland Yard said officers had "contacted a number of individuals who have stated they intend to take part in protest on Saturday".
London's mayor said people were entitled to demonstrate but warned police would take a tough line on any violence.
"We live in a democracy where people are entitled to protest, they are entitled to have fun and do what they want," Johnson told LBC radio on Friday when asked about the Trafalgar Square event.
"What they can't do is, I think, is use the death of an elderly person to begin riot or affray or that kind of thing."
-- Thatcher 'left detailed instructions' --
Police are mounting a major security operation for Wednesday's funeral, when Thatcher's coffin will be taken to St Paul's Cathedral through streets lined with members of the armed forces.
The ceremony itself will be carried out in line with strict instructions left by the former premier herself, according to the Telegraph.
She requested that Cameron, as prime minister of the day, read a passage from the Bible but did not want any political eulogy, the newspaper reported. She also asked that her favourite hymns be sung.
In a reflection of her strong partnership with Ronald Reagan in fighting communism in the Cold War, the guest list includes all the surviving US presidents.
But several towering politicians from Thatcher's era, including former Soviet leader Mikhail Gorbachev, German ex-chancellor Helmut Kohl and Reagan's widow Nancy, are not attending due to ill health.
The guests will join members of Thatcher's family including her 59-year-old twins, Mark and Carol, who were reunited on Friday evening for the first time since their mother's death.
They were both out of the country when Thatcher died on Monday following a stroke at London's Ritz hotel, aged 87.
In a statement outside her mother's former London home on Saturday, Carol Thatcher said she was bracing for a "tough and tearful week".
"I feel like anyone else who has just lost a second parent," she said. Denis Thatcher, her mother's devoted husband for 52 years, died in 2003.
Carol Thatcher hailed this week's "magnificent" tributes from world leaders including US President Barack Obama as proof that her mother made her mark on history.
"But I know that this is going to be a tough and tearful week, even for the daughter of the Iron Lady," she added.
Thatcher, a Conservative, was Britain's only female prime minister and the longest serving British premier of the 20th century, holding office from 1979 to 1990.
Her death has sparked fierce debate about her legacy in Britain.
Her admirers, who credit her with helping to end the Cold War and reinvigorating the British economy after decades of decline, are campaigning for a public memorial.
But left-wing opponents accuse her of pushing a ruthlessly individualistic agenda and putting millions out of work with her radical free-market reforms.