Police risked controversy today 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 will 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.
He called on police to show greater flexibility, including allowing a controlled release of people.
Hundreds of people chanted "let us out" as a line of police officers reduced the size of the Whitehall pen.
Many argued the police were punishing everyone, rather than the handful of troublemakers.
At some point, all over the western world, we're going to wake up, look around and go "Hey, where did this police state come from?"