Over 50,000 people brought Westminster to a standstill with a peaceful march past Parliament to protest against the proposal to increase tuition fees to up to £9,000 a year.
But the demonstration turned nasty when a crowd smashed its way into the Conservative Party's headquarters in Millbank, cheered on by hundreds more outside.
The ferocity of the protest ended the high hopes of a new era of consensus politics promised by David Cameron when he took office exactly six months ago.
The perpetrators of this chaos were very young, surprisingly well-mannered and rather middle-class. Some of those hurling insults or wooden placard polls at riot police were 15 and 16-years-olds who had bunked off school and now stand a chance of getting a criminal record before they reach university. Last night there had been 35 arrests.
Terrified Tory staff barricaded themselves in their offices as demonstrators roamed the building. Furniture was thrown through windows, the interior was trashed and a ceiling was pulled down. A fire-extinguisher was thrown off the roof of the eight-storey building at police in the crowded courtyard below.
Slogans such as “Tory scum”, and others more obscene, were scrawled across walls in paint and marker pen. Lights were ripped down and placards were burnt.
Water fire-extinguishers were also let off from the roof. Eight people, including three police officers, were taken to hospital.
Riot officers were outnumbered, with 30 desperately trying to hold their line and protect the building despite a steady bombardment. Reinforcement attempts were made as darkness fell but they were driven back by protesters.
The Metropolitan Police Commissioner Sir Paul Stephenson admitted afterwards: “It's not acceptable. It's an embarrassment for London and for us.”
Some of the protesters blamed the confrontation on police. Oscar, an 18-year-old sixth-form politics student, claimed: “It was disgusting, man. They got their batons out and were knocking people to the floor. One girl was hit on the head. It's just made people more angry.”
Earlier, a peaceful demonstration disrupted Prime Minister's Questions, with an estimated 52,000 students cramming into Whitehall, bringing London to a standstill. Chanting rang around Westminster as Nick Clegg tried to defend the coalition's cuts.
Labour MPs lined up to taunt Mr Clegg about the pledge all 57 Lib Dem MPs signed before the General Election, vowing to oppose any increase in tuition fees.
Mr Clegg said: “I have been entirely open about the fact that we have not been able to deliver the policy that we held in opposition.”
The violence was condemned by organisers. Aaron Porter, president of the National Union of Students, accused a “small minority” of having “hijacked” the event.
One well-spoken 16-year-old from Worcestershire, named Alex, was up on the roof with a scarf across his face in a feeble attempt to hide his identity. He thought the violence was justified “as long as no one gets hurt”. “This is f****** amazing,” he said. “You should go up on the roof. It's chaos up there — they've grafittied all over the walls. I want to study journalism when I finish school, if I can afford it. People are p****d off.”
Andrew Speake (23), a Chinese studies student, called the violence “a necessary evil”.
The protesters, reputedly organised by a revolutionary group from Leeds, in a statement, said: “We are occupying the roof in opposition to the marketisation of education pushed through by the coalition Government, and the system they are pushing through of helping the rich and attacking the poor. We call for direct action to oppose these cuts. This is only the beginning of the resistance.”