Sir Paul Stephenson: The Tamil protest has prevented policing in other areas
The Metropolitan Police says it has spent almost £8m monitoring the Tamil protest at Parliament Square.
Metropolitan Police Commissioner Sir Paul Stephenson said the protests, which had have gone on for 43 days, had stretched the force's resources.
Speaking to the the Home Affairs Committee, he said the force had spent "just short of £8m" - more than the £7.2m spent on G20 policing.
Ten people were arrested on Tuesday as police cleared protesters off roads.
Demonstrators had been trying to draw attention to the plight of Tamils in Sri Lanka, where government forces have been fighting separatist rebels.
About half of the total spent policing the demonstration - £3.72 million - was from additional policing costs, including overtime, the Met said.
"We have to provide such a level of resources that it is reducing policing on the streets of London," said Sir Paul.
"Whatever the rights and wrongs of any demonstration, it has to be said that policing that demonstration is a huge drain."
Sir Paul added: "This is damaging the Met's performance and does lead to lack of policing on the streets of London."
Commander Bob Broadhurst said police were having problems dealing with the demonstration because there was not a single spokesperson for the protesters.
The Metropolitan Police said officers had cleared the roads around Parliament Square at about 0015 BST. About 25 officers were injured during the operation.
Ambulance crews said three police officers and five protesters were taken to hospital.