There have been violent scenes as tens of thousands of people protested against plans to treble tuition fees and cut university funding in England.
Demonstrators stormed a building in Westminster housing the Conservative Party headquarters, smashed windows and got on to the roof.
Outside, a crowd of thousands surged as placards and banners were set on fire and missiles were thrown.
Student leaders condemned the violence as "despicable".
They say about 50,000 people took part in a march through Westminster earlier.
A stand-off is still taking place between about two dozen demonstrators and the police, with 32 people having been arrested so far.
According to Scotland Yard, 14 people have been injured, including seven police officers. No-one was seriously hurt.
The vast majority of demonstrators had been peaceful, a statement said, but "a small minority" had damaged property.
At one point, a fire extinguisher was reported to have been thrown from the roof.
The police have faced accusations that they did not have enough officers on duty when the clashes erupted.
The Metropolitan Police Commissioner Sir Paul Stephenson said the force should have anticipated the level of violence "better".
"It's not acceptable. It's an embarrassment for London and for us," he said.
"I am determined to have a thorough investigation into this matter," he added.
London Mayor Boris Johnson said: "I am appalled that a small minority have today shamefully abused their right to protest.
"This is intolerable and all those involved will be pursued and they will face the full force of the law.
"The Metropolitan Police Commissioner has assured me that there will be a vigorous post-incident investigation."
BBC News correspondent Mike Sergeant, who is at the scene, said protesters on the roof had thrown liquids down and a female police officer had been injured.
At 1700 GMT he said the police had "largely taken control" of the building; he had seen some protesters escorted out by officers and the crowds outside were gradually dispersing.
One of the protesters who got on to the roof was Manchester student Emily Parks.
"It shows how angry people are," she told BBC News.
"Why is our education being cut? Why are tuition fees going up here when in other parts people have free education? People have felt the need to take matters into their own hands."
Conservative Party chairman Baroness Warsi was inside the building while the protest was taking place.
She said police had responded "in the circumstances that they felt best.
"This was clearly a protest where people had a legitimate right to protest on issues that they felt very strongly, and it is a shame that a small minority of those protesters ruined it for the rest of them."
Demonstrators were also cleared from outside the Liberal Democrat headquarters, where a car window has been smashed.
Elsewhere, the massive rally had passed off peacefully.
Hundreds of coachloads of students and lecturers had travelled to London from across England for the demonstration in Whitehall, with 2,000 students also travelling from Wales.
National Union of Students president Aaron Porter condemned the violence as "despicable".
"This was not part of our plan," he said.
"This action was by others who have come out and used this opportunity to hijack a peaceful protest."
The NUS is threatening to try to unseat Liberal Democrat MPs who go back on pre-election pledges they made to oppose any rise in tuition fees.
Higher education funding is being cut by 40% - with teaching grants being all but wiped out except for science and maths.
The government expects the costs of teaching other courses to be funded by tuition fees.
It proposes that tuition fees should rise from 2012.
The plan is for a lower cap at £6,000, with universities able to charge up to £9,000 - triple the current cap - in "exceptional circumstances".
Ministers insist their plans offer a "fair deal for students".
Earlier on Wednesday, at Question Time in the Commons, Deputy Prime Minister Nick Clegg had a fiery exchange with Labour's Harriet Harman over fees.
He was accused of hypocrisy, because the Liberal Democrats opposed tuition fees in the election run-up.
But he said Labour had made U-turns itself over fees, which it brought in in 1997, and said the party had no clear alternative policy.
Ms Harman said Nick Clegg was "going along with a Tory plan - to shove the cost of higher education on to students and their families".
Twice, Mr Clegg sidestepped her request that he specify the size of the cut to university teaching grants - a figure she said was 80%.
Aaron Porter said students will attempt to force a by-election in the constituencies of MPs who renege on a pre-election pledge to oppose any hike.
In a speech in June, Mr Clegg said the coalition would bring in a right for voters to re-call their MP and force a by-election if he or she was found to have been engaged in "serious wrongdoing".
Students plan to make Mr Clegg their first target - and say they will be collecting signatures in his constituency on Monday.
But as yet, no laws have been brought in to make such "re-calls" possible.
Universities Minister David Willetts said the new system would be fairer than the present one, offering more help to the poorest students.
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.
Among the crowds at the rally in London were about 400 students from Oxford.
Oxford University Student Union President David Barclay said: "This is the day a generation of politicians learn that though they might forget their promises, students won't.