The arrests were made as police tried to move protestors off the road at one of the British capital's busiest points, after their demonstration had closed it to traffic for several hours.
"The majority (of arrests) were for wilful obstruction and there were also arrests for violent disorder and assaulting police," a spokesman for London's Metropolitan Police said, adding that the demonstration had been cleared.
Thousands of protestors chanting, carrying banners saying "Stop the war, start the talks" and beating drums sat down in the streets earlier Monday in protest at deaths in Sri Lanka on Saturday and Sunday.
They had pushed through police lines keeping them on grass in the middle of Parliament Square and sat down in the road on two sides of it, blocking access to Whitehall near to Prime Minister Gordon Brown's office in Downing Street.
One protestor, student Bharathy Maheswaram, said: "All these people have relatives, friends and family out there so hearing that (the killing o