A student demonstration against higher tuition fees descended into chaos, with dozens of people arrested after violent clashes with police.
Scotland Yard admitted it was unprepared for the trouble caused by a minority of the protesters, who stormed the building which houses the Conservative Party headquarters in one of the biggest demonstrations seen on London's streets in recent years.
The Metropolitan Police said 35 people are now in custody after being arrested for offences including criminal damage and trespass.
Thousands of people flooded the streets of Westminster for the protest, which led to 14 being treated in hospital, seven of them police officers. None of the injuries were thought to be serious.
Police said they had anticipated the scale of the event but not the violence. They initially drafted in 225 officers but admitted the number had to be "vastly increased" as the angry scenes unfolded.
Met Police Commissioner Sir Paul Stephenson said: "It's not acceptable. It's an embarrassment for London and for us. I am determined to have a thorough investigation into this matter. We didn't expect this level of violence."
At the height of the protest, windows at Millbank Tower in central London were smashed with a hammer, with the demonstrators who got inside the building's atrium trying to pull down the remaining sheets of glass.
About 50 people got on to the roof of the tower and were seen dropping missiles, including a large metal fire extinguisher, on to the riot police below. Protesters on the ground threw sticks and other items at officers, one of whom was led away with a bleeding wound to her head.
Hundreds of workers were evacuated from Millbank Tower, which also houses other organisations and Government agencies. Baroness Warsi, co-chairman of the Conservative Party, had been in 30 Millbank with her staff.
An estimated 50,000 students and lecturers took part in the march and the unions who organised it blaming troublemakers for "hijacking" the event, with National Union of Students president Aaron Porter saying he believes the violent protesters had planned their actions in advance.