Pro-Tamil protesters blocked traffic during Monday's protests
Ten people were arrested for public order offences as police ended a Tamil supporter protest which had blocked roads around Parliament since April.
Initially about 2,000 people gathered for a sit-down protest over the plight of Tamils in Sri Lanka on a road opposite the Houses of Parliament.
The Metropolitan (Met) Police said officers had cleared the roads around Parliament Square at about 0015 BST.
Ambulance crews said three police and five protesters were taken to hospital.
Demonstrators had been trying to draw attention to the plight of Tamils in Sri Lanka.
Between 50 and 100 protesters are now back behind barriers on Parliament Square itself and no longer causing a traffic hazard, a Met police spokesman added.
Sir Paul Stephenson: The Tamil protest has prevented policing in other areas
He also said 21 police officers had been treated for minor injuries, with two requiring hospital treatment, but they were not believed to be seriously hurt.
However, London Ambulance Service said it had treated two police officers at the scene and three had gone to hospital.
Six protesters were treated at the scene, it said.
Met Police Commissioner Sir Paul Stephenson said on Tuesday policing the protest was stretching the Met's resources.
"Whatever the rights and wrongs of any demonstration it does have to be said at this moment in time policing that demonstration is a huge drain on the resources that should be available to Londoners."
"This is damaging the Met's performance and does lead to lack of policing on the streets of London."
The protest came as military leaders in Sri Lanka said they had crushed the rebel Tamil Tigers' 26-year insurgency aimed at securing a separate state in the north and east of the country.
The military also said the leader of the Tamil Tigers, Velupillai Prabhakaran, had been killed.
At the height of Monday's protest, police helicopters circled above Parliament Square and there was a large presence on the ground as traffic in the area was diverted.
Many of those involved in the demonstration wore a white cloth around their heads and arms as a sign of solidarity.
Student Kulan Selvan, 32, from Catford, London, said: "There is a strong feeling of real anger here.
"For us the war is not over and we simply do not believe reports that the Tamil leader has been killed.
"We are deeply concerned for the people in our homeland. We believe thousands have been killed.
"The Sri Lankan government have refused to let the media in so we just don't know how bad it really is there."
He urged the media to give more prominent coverage to the conflicts in Sri Lanka.
Some banners urged US President Barack Obama to send aid ships to Sri Lanka.