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.
Ben Mann, 24, a London University student, said: "It's not good. It makes people more angry. I don't understand how they have the right to hold people in one place.
"It really angered people when they did this at the G20 protests. A policeman just told me this was the end of protests as we know it, which was pretty scary."
Tom, a 23-year-old Sussex University student who didn't want to give his surname, said: "They're trying to deter people from protesting.
"They're not accusing us of any crimes, so why have they done it? This is preventing us getting our message across."
Sophie Battams, 17, from Dagenham, Essex, said: "The kettling is causing the violence.
"If you put a lot of angry people in one area, it will escalate to this."
Rachel Tijani, 18, also from Dagenham, said: "It's a self-fulfilling prophecy. If they coop people up like caged animals, they'll act like caged animals.
"It was peaceful at first, then it got violent as people wanted to make their point. I just want to go home now."
The containment tactic was first used to hold off demonstrators in Oxford Circus on Mayday 2001.
It was declared legal by the House of Lords after a series of court challenges by protest groups.
But the Joint Committee on Human Rights (JCHR) found the controversial technique did not respect the human rights of protesters.
Members said the Met did "not give sufficient weight" to the rights of individuals contained within police cordons.
Inside the containment area, Barrie Buxton, 68, from Gravesend, Kent, said: "I'm here as a gesture of solidarity.
"The tables have been turned - people used to do sit-ins to annoy people, but now we've been penned in which is annoying us.
"This is a problem that needs to be solved, there is an urge to demonstrate but peaceful demonstration gets nowhere."
As it got darker, the shouts of the crowd got louder as they called to be released. Pockets of demonstrators pushed forward and were held back by police.
Plastic riot shields taken from the police van were set alight as makeshift fires.
At least one protester started to do his homework as he waited to be released from the cordon.
James Welch, of Liberty, said: "Nobody should condone the violent actions of a small group of troublemakers who try to hijack a peaceful protest.
"But as we saw at the G20, kettling is a very dangerous police tactic.
"It traps the innocent and vulnerable with the guilty and hostile, spreads alarm and frustration and risks making a bad situation a lot worse."
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