The clear-up following violent protests against tuition fee rises in central London could cost more than £50,000, the mayor's office has said.
Boris Johnson's spokeswoman said the protests could hit London's global reputation ahead of the 2012 Olympics.
So far, five tonnes of debris and graffiti on statues in Parliament Square have been cleared.
The Christmas Tree in Trafalgar Square - an annual gift from Norway - was also targeted by protesters.
The protests over MPs voting to increase university tuition fees in England to up to £9,000 a year turned violent on Thursday.
Some demonstrators started fires, sprayed graffiti and smashed the windows of the Treasury building.
'Insult to democracy'
A car carrying the the Prince of Wales and Duchess of Cornwall was also attacked while shops in Regent Street were also targeted.
In Trafalgar Square, some activists tried to set fire to the Christmas Tree, which has been an annual gift from Norway since 1947 in thanks for Britain's support during World War II.
The 20m high Norwegian spruce is about 60 years old and was selected from the forests surrounding Oslo.
Westminster council's operations director Rob McAlister, who was overseeing the cleaning operation, said the clear-up began at 2200 GMT.
Cleaners from the Greater London Authority, which looks after the upkeep of Parliament Square, also worked through the night.
The mayor's spokeswoman said: "Repairing the extensive damage caused to both Parliament and Trafalgar Squares could cost in excess of £50,000, which is an unacceptable burden on the public purse.
"Parliament Square Gardens was subjected to sustained and violent attacks by some protesters, intent on mayhem, who ripped down fencing, set fire to benches and in a wanton insult to democracy, defaced the statues of some of the greatest protectors of the very freedoms and privileges those intent on causing damage yesterday abused.
"The protesters who chose to wreak havoc on our city should also be aware of the potential greater cost to London's global reputation as we prepare to host the world during the 2012 Games."
Mr McAlister said: "A lot of it is graffiti, a lot of it is paint, lots of utility cones and barriers were used in the protests.
"We have removed five tonnes' worth of debris associated with the protests throughout the city so that's quite a lot of stuff that we've had to move overnight."
Condemning the "mayhem", Westminster Council Leader Colin Barrow said staff were diverted from their usual work for the clear-up.
National Union of Students (NUS) president Aaron Porter said violence at protests was "morally wrong" and "counter-productive", but he pledged the fees campaign would continue.
He said: "The past few months have engaged many more people of all ages in education campaigns. We have the public on our side, but every scene of violence erodes that vital support.
"We will continue to work with the police to ensure that the rights of citizens to protest are protected and to find the best ways to ensure the safety of protesters and the public."