Around 100 police officers were in Soho, central London, where protesters with banners were believed to be occupying a former police station in Beak Street.
The move by police came as protesters from the group Stop G8 planned a “Carnival Against Capitalism” in central London, ahead of the G8 conference in Belfast, Northern Ireland.
Businesses in central London have reportedly warned staff to take precautions after similar protests in recent years led to violent clashes with police.
Westminster City Council cabinet member for city management Ed Argar said: “Our warden teams will be working with police to manage the streets and we also have clean-up crews on standby.
“Everyone respects the right to legitimate protest and I hope this will be a day without incident.
There were reports on Twitter of strong police presences in the Oxford Circus and Regent Street area of central London and Golden Square, close to Beak Street.
Another Ed McClaran, @edmcclaran, tweeted: “Big G8 protest outside outside our offices today. People chaining themselves to railings. Why they are in the West End and not the City, I don’t know.”
The leaders of the world’s eight wealthiest countries, including Russian President Vladimir Putin and German Chancellor Angela Merkel, are due to meet at the luxury Lough Erne resort in Co Fermanagh for the two-day conference next week.
US President Barack Obama is to visit parts of Belfast ahead of the summit.