Police must prepare for more lawless demonstrations because the "game has changed", Britain's most senior officer said today.
Sir Paul Stephenson said his officers failed to correctly assess the mood of protesters in a new political climate before the Millbank riot two weeks ago.
But the Metropolitan Police boss said they did not make that mistake yesterday as clashes once again marred a mass march against rising student tuition fees.
The London force deployed more than 800 officers, almost four times as many as on November 10, as police prepared for a fresh wave of violence and disorder.
Potential targets for occupation, including the headquarters of the Liberal Democrats, were encircled by officers as reserves surrounded Trafalgar Square.
Speaking at a meeting of the Metropolitan Police Authority (MPA) at City Hall, Sir Paul said the "likelihood is for more disorder on our streets".
He said: "We have been going through a period where we have not seen that sort of violent disorder.
"We had dealt with student organisers before and I think we based it too much on history. If we follow an intelligence-based model that stops you doing that.
"Obviously you realise the game has changed. Regrettably, the game has changed and we must act."
The senior officer defended the decision to "kettle" thousands of protesters, including many school pupils, in Whitehall, close to the entrance to Downing Street.
He said when several large-scale marches linked up in Whitehall police wanted to stop them getting into Parliament Square and beyond.
The meeting heard a smaller group of known troublemakers tried and failed to target the Lib Dem Cowley Street headquarters earlier in the day.
Sir Paul said in recent years the Met has been reducing the numbers of officers and other resources deployed to tackle demonstrations.
He said: "Regrettably we are going to have to review that. We are going to have to take a more cautious approach."
Speaking about the clashes yesterday, Sir Paul said: "The bottom line is we did not get it right two weeks ago and in my opinion we did get it right yesterday.
"We did put an enormous amount more assets out yesterday but we did get it right."
Sir Paul said out of the 35 people arrested yesterday, nine were held for vandalising and looting a police carrier left stranded in the centre of Whitehall.
He added that seven officers were injured, including a woman who suffered a broken bone in her hand and a man with "soft tissue damage" to his leg.
A further 11 members of the public were treated by paramedics as thousands of people descended on Westminster.
Several landmark buildings, including those in Whitehall, were "locked down" over fears protesters were planning to occupy them by force.
Police said protesters wore masks, ripped up roadworks, destroyed a bus shelter, threw smoke canisters and ignited aerosols, burning some people's faces.
Sir Paul said there were no police officers in the vandalised police carrier and it was abandoned because of the risk of getting it back.
He added: "It is beyond irritating, it is disgraceful, but had we deployed to that carrier we would have achieved nothing. We could have injured protesters and police officers.
"The decision was made to leave it there and evidence gather and deal with the criminals later, which is what we have done."
Speaking about the decision to pen in protesters, Sir Paul said there were fears some would try to occupy the Liberal Democrat headquarters.
He said: "Given their intentions and the criminal damage seen in Parliament Street and Whitehall, a containment, also known by another phrase, was authorised at 1pm.
"The command team was in no doubt if protesters were given free reign in Parliament Square and the Government security zone, they would have tried to enter premises by force."
Sir Paul said police wanted to release those held in the pen earlier, but could not because of renewed violent clashes involving people outside the area.
He added that the process of photographing and identifying suspects before they were allowed to leave was "frustratingly slow".
The meeting heard that clashes continued late into the evening with sit-down demonstrations on Westminster Bridge and in Parliament Square.
A group of students staged a protest at the London constituency office of Simon Hughes, the deputy leader of the Liberal Democrat party.
Police were called to the offices after around 40 students stormed in, chanting: "No ifs, no buts, no education cuts."
One of the students said they would remain in the office until Mr Hughes turned up.
Mr Hughes said: "Some demonstrators attempted to enter our Liberal Democrat office in Bermondsey this afternoon but have been prevented from doing so. The situation is under control and the police are in attendance.
"If anybody thinks that the way to persuade me or my colleagues to their point of view is to disrupt the work done by my staff or by me looking after my constituents then they are making a foolish mistake.
"Regularly and rationally, privately and publicly I engage in debate with many students and other people about higher education and many other issues.
"I will continue working for the best possible deal for the next generation of over 16s who want apprenticeships, training and further or higher education."Reuse content