Hundreds of student protesters have clashed with police during a rally against higher tuition fees in London. They broke into Conservative HQ, smashing windows and wrecking a lobby.
They got onto the roof, lit fires and threw missiles at police - who have admitted to Channel 4 News they were "embarrassed" at being outnumbered.
Dozens have been arrrested and ten people needed hospital treatment after the violent clashes during which demonstrators chanted "Tory scum" at plans to increase the cap on university tuition fees.
Student leaders are furious their protest was hijacked - but hundreds if not thousands of marchers abandoned the official route of the protest and effectively joined the siege.
Met Police Commissioner Sir Paul Stephenson said: "It's not acceptable. It's an embarrassment for London and for us."
He added: "I am determined to have a thorough investigation into this matter. We didn't expect this level of violence."
Hundreds of people were evacuated when trouble flared at Millbank Tower in the heart of Westminster.
One eyewitness who works in the building but did not want to be named said: "The fire alarm went off and every one was evacuated from the building. There are hundreds of students outside.
"We were told that it was a false fire alarm because students were throwing smoke bombs into the building."
Thames House, the MI5 headquarters close by, was sealed with heavy metal doors and police were guarding the rear exits.
This is only the beginning of the resistance to the destruction of our education system and public services. Protesters' statement
Dozens of protesters climbed onto the roof of the building next to Millbank Tower. One worker said individual floors were taking the decision to send staff home early.
The protesters in the Tory HQ building and on the roof released a statement which said: "We oppose all cuts and we stand in solidarity with public sector workers, and all poor, disabled, elderly and working people."
Chief Correspondent Alex Thomson blogs from the wrecked lobby at Millbank Tower:
Two quite different actions taking place now.
Inside the smashed up reception area of 30 Millbank, police transits come and go, transporting away protesters arrested inside Tory Party HQ.
They edge gingerly out in the arms of riot police past the smashed doors and wrecked windows across the courtyard filled with leaflets, smashed placards, cans and the ash of small fires.
Thirty yards away on London's embankment, still closed to traffic, perhaps a couple of hundred demonstrators remain corralled.
But there is movement. One by one they are led out to an area where several groups of police officers are videoing them, asking questions, and noting down details.
Thus the kettle is being emptied at an immensely slow pace. This process is likely to take several hours but the police have not told us under what powers they are able to do this. I have seen the same operation at G20 and a number of other protests.
What is clear is that - as one might expect - these students did their homework, clearly outflanking the police in reaching Millbank in numbers before they did. I can see both sides making something of this - the Police Federation may well use it as ammunition for their argument for funding, just like the students.
One curious fact though today, I spoke to three different policemen in very different areas, all of whom said there would be a lot more of this and expressed considerable sympathy with what happened here today.
That sentiment echoed too by a 54-year-old office manager of a charity, who looked at the damage here and said that phrase we have heard so much today: "this is only the beginning".
These students of cojurse will not remember, and nor can I, the events of May '68 in Paris and the student slogan then: "C'est n'est tu'un debut" - this is only a start.
They will not even remember perhaps the poll tax riots in Trafalgar Square where there was a genuine riot and widespread damage.
The point being that violent demonstration sometimes works. A short time of after that event the hated poll tax and indeed Margaret Thatcher were history.