Student protests: Police 'struck wrong balance'
Police "struck the wrong balance" when preparing for Wednesday's tuition fee protests which were marred by violence, the policing minister has said.
Nick Herbert said police would learn lessons after a "hard core" minority stormed the Conservatives' London HQ. Dozens were injured and 50 arrested.
One protester who threw a fire extinguisher from the roof could face an attempted murder charge.
The Met Police has announced an inquiry into the handling of the student march.
Those arrested - mostly for criminal damage and aggravated trespass - have been released on police bail until February.
Chairman of the Police Federation, Paul McKeever, told the BBC: "The person who threw the canister off the roof and into the crowd of people needs to be aware that such potentially lethal actions have consequences.
"The penalty for this type of incident is the charge of attempted murder."
Thousands of students had taken to London's streets to join the demonstration against the plan to lift the cap on university tuition fees to £9,000.
About 2,000 split from the main march to gather outside 30 Millbank, the Conservative headquarters in Westminster, where windows were smashed, fires lit and missiles thrown at police, Mr Herbert said.
"Many appeared not to be directly involved in violence. It is now clear that a small hard core within this group were intent on violence," the policing minister told the Commons.
About 225 officers had originally been deployed to police the march, the minister said, although a further 225 were called in as the situation developed.
"The police have to strike a balance between dealing promptly and robustly with violent and unlawful activity on the one hand, and allowing the right to protest on the other," Mr Herbert said.
"Clearly in this case the balance was wrong but these are difficult decisions and they are not taken lightly."
Earlier Mr Cameron called the protests "unacceptable" and said there had not been enough officers to control the crowds.
He called for the "full force of the law" to be used against those who had been violent.
Some protesters had broken into the Millbank building, although hundreds of workers, including Tory party staff, had already been evacuated.
Mr Cameron said he was watching what had happened on television from Seoul, where he was attending a G20 summit.
"I was worried for the safety of people in the building because I know people who work in there, not just the Conservative Party, but other offices as well, and so I was on the telephone", he said.
He said protests were a part of democracy but violence and law-breaking was not.
Metropolitan Police Commissioner Sir Paul Stephenson has said the police should have better prepared and called Wednesday's events "an embarrassment".
He said: "It's not acceptable. It's an embarrassment for London and for us."
Police are continuing to examine CCTV footage of the incident.
The National Union of Students (NUS) said about 50,000 people joined the demonstration, but according to Scotland Yard, no trouble was anticipated.
Shadow home secretary Ed Balls described the vandalism as "completely unacceptable".
"It was perpetrated by a small minority of thugs who hijacked what was planned to be a legitimate and peaceful demonstration, and in so doing denied tens of thousands of students and lecturers the right to have their voices properly heard."
However, he said the investigation should look at whether there was intelligence to suggest pre-planned violence, whether sufficient officers and back-up were deployed, and how operational decisions were made about which buildings to protect.
And shadow business secretary John Denham said the students had a legitimate cause and needed to be listened to.
He said: "Those parents and the students who were there yesterday have a legitimate cause that needs to be taken up and debated. We mustn't allow the despicable action of some protesters to divert attention from a real issue."
Hundreds of coachloads of students and lecturers travelled to London from across England, Wales and Scotland for the demonstration in Whitehall.
As well as higher fees, they were protesting against plans to cut higher education funding by 40% and to all but wipe out teaching grants except for science and maths.
However Mr Cameron said he would not abandon his plan to reform tuition fees to allow some institutions to charge up to £9,000 a year.
NUS president Aaron Porter said he believed members had "lost a lot of public sympathy" because of what happened.
But Clare Solomon, president of the University of London Student Union, predicted a growing wave of similar protests in coming months.
London Mayor Boris Johnson said he also hoped those responsible for violence "paid a serious price for their actions".
Asked about his decision to sign an NUS pledge promising to fight any rise in fees, Mr Clegg told ITV1's Daybreak: "I should have been more careful perhaps in signing that pledge. At the time I thought I could do it."
But Shadow Commons leader Hilary Benn said Mr Clegg knew exactly what he was doing.
"Before the election [the Lib Dems] made everything of their pledge to vote against the lifting of tuition fees, after the election they couldn't dump it fast enough."
Under the coalition's plans, students would not have to pay anything "up front" and as graduates, would only have to pay back their tuition fee loans once they were earning £21,000 or more.
But the NUS and other opponents say the prospect of such large debts will deter young people from poorer backgrounds from going to university.