As a handbag-toting Thatcher effigy made its way down the stairs in front of the National Gallery, the crowd erupted into cries of "Maggie! Maggie! Maggie! Dead! Dead! Dead!" and sang lyrics from the "Wizard of Oz" ditty "Ding Dong! The Witch is Dead."
Photographs and video footage showed hundreds of people clutching their umbrellas in the rain between Nelson's Column and the National Gallery on the square. Official crowd estimates weren't immediately available.
Britons remain deeply divided over Thatcher, who died Monday aged 87, and the debate over her legacy has revived the strong feelings that marked her more than decade-long term in office.
Widely respected on the right for reviving Britain's economic fortunes and besting Argentina in a war over the Falklands, Thatcher is reviled by some on the left for her bruising confrontation with the country's union movement and her perceived indifference to the country's working class.