Police have risked controversy as they "kettled" several hundred protesters after clashes during student tuition fee protests.
Demonstrators were penned in by riot squad officers in Whitehall after a stranded police van was attacked and vandalised.
Scotland Yard said it would bring in portable toilets and water but said it was unclear how long people would have to remain.
The tactic, which police call "containment", was heavily criticised after up to 5,000 people were held during G20 protests in April last year.
Denis O'Connor, HM Inspector of Constabulary, said the move carries risks, particularly if vulnerable people are caught up inside.
Hundreds of people chanted "let us out" as a line of police officers reduced the size of the Whitehall pen.
The demonstration followed a day of action two weeks ago that saw 60 arrested and dozens injured when a riot broke out at the Conservative Party headquarters.
Marches have been held across England and Wales, with school and college students joining the action.
Cities including London, Birmingham and Manchester were holding rallies, with university workers protesting alongside students.