David Cameron has condemned the violence that broke out during protests over tuition fees after the Met Police announced an investigation into how it was handled.
The prime minister said the clashes in central London, which led to 50 arrests and 14 injuries, were "unacceptable".
He praised the "brave" officers who tried to control the crowds, but said "there weren't enough of them".
Met chief Sir Paul Stephenson called Wednesday's events "an embarrassment".
Seven police officers were among those hurt during the protests outside Conservative Party headquarters at Millbank, Westminster, on Wednesday.
Windows were smashed, fires lit and missiles thrown at police after a group of protesters broke away from the main demonstration against a planned rise in university fees.
Some broke into the building itself, although hundreds of workers, including Tory Party staff, had already been evacuated.
'Thin blue line'
Ahead of a statement in the Commons on the violence Tory MPs Nadine Dorries and Peter Bottomley warned that people could have died. Ms Dorries accused NUS student officials of "egging on" the crowd while Mr Bottomley likened the scenes to the Heysel stadium disaster.
Mr Cameron said he had watched events unfold from Seoul, where he was attending a G20 summit, and had been concerned.
"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."
Sir Paul apologised to those left inside the building for their "traumatic experience" and said the Met should have been better prepared.
Scotland Yard said all 50 of those arrested had now been released on bail until February while police continued inquiries and looked at CCTV evidence. The majority were being held for criminal damage and aggravated trespass.
The National Union of Students (NUS) said about 50,000 people joined the demonstration, but according to Scotland Yard, only 225 officers were initially drafted in to police it because no trouble was anticipated.
"It's not acceptable. It's an embarrassment for London and for us," Sir Paul said.
"I think we've also got to ask ourselves some questions. This level of violence was largely unexpected and what lessons can we learn for the future."
Mr Cameron said the situation had been "extremely serious" and welcomed the decision to hold an inquiry.
"I could see a line, a thin blue line of extremely brave police officers, trying to hold back a bunch of people who were intent on violence and destruction.
"They were very brave those police officers, but as the police themselves have said there weren't enough of them and the police response needs to reflect that, so I'm very glad that the Met Police commissioner has said what he said."
Hundreds of coachloads of students and lecturers travelled to London from across England for the demonstration in Whitehall, with 2,000 students also travelling from Wales and a further 2,000 from Scotland.
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.
But the PM said he would not abandon his plan to reform tuition fees and allow some institutions to charge up to £9,000 a year.
Asked whether the protests reminded him of the sort of unrest seen under Margaret Thatcher's Conservative government in the 1980s, Mr Cameron said: "There have been protests, both peaceful protests and sometimes protests that have turned quite nasty, under all governments, so I don't see it like that."
NUS president Aaron Porter told BBC Breakfast his members had "lost a lot of public sympathy" because of what happened.
"What we had done was assemble 50,000 students which I'm sure would have got a hell of a lot of attention and would have sent a clear message to government," he said.
"But if we're now having to spend time talking about the rights and wrongs of violence and criminal damage, actually in many respects I think it undermines our argument rather than allowing us to concentrate on the devastation to our universities and colleges."
But Clare Solomon, president of the University of London Student Union, said she had "no problem with direct actions or occupation" and predicted a growing wave of similar protests in coming months.
She added: "These were a few windows of the Tory Party headquarters - what they're doing to our education is absolutely millions… and they want to complain about a few windows."
Former Flying Squad commander John O'Connor accused the Met of having "no tactics and nowhere near enough people" at the scene.
"It's an absolute disgrace," he told the BBC.
"I don't understand how anybody could have sat there planning that event and not taken the contingency plan into consideration that this is likely to erupt into violence."
But London's deputy mayor Kit Malthouse, who has responsibility for the capital's policing, said "a large number of meetings" had been held with the NUS before the protests and a lot of intelligence gathered.
"None of that gave any cause for us to believe that there was going to be a significant problem," he added.
Police Federation spokesman Paul McKeevor said officers were "not a side in this dispute" and added that when it came to using force against demonstrators: "We're damned if we do and damned if we don't."
At prime minister's questions in the Commons on Wednesday, Deputy Prime Minister Nick Clegg was accused of hypocrisy by Labour because the Liberal Democrats had promised to scrap tuition fees altogether in their election manifesto.
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."
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.