Thatcher's most strident critics had long vowed to hold a party in central London on the Saturday following the former leader's passing.
Photographs and video footage appeared to show a few hundred people clutching their umbrellas in the rain between Nelson's Column and London's National Gallery.
Britons remain deeply divided over Thatcher, who died Monday aged 87, and the debate over her legacy has aroused strong feelings in London.
Get Breaking news, live coverage, and Latest News from India and around the world on NDTV.com. Catch all the Live TV action on NDTV 24x7 and NDTV India. Like us on Facebook or follow us on Twitter and Instagram for latest news and live news updates.