More than 25 people were arrested as fights broke out marring a largely peaceful demonstration.
The main scuffles were around the Prime Minsters address, Downing Street, where police were forced to set up a human shield, hemming in about 40 people.
Many protesters wore face masks and sunglasses to hide their identities.
When troublemakers were eventually cleared, police tried to clear Whitehall to allow traffic to move more freely but a large group of protesters sat a stage in before they were removed one by one.
The demonstration was one of many taking place simultaneously in many UK cities by public sector workers furious at Government austerity plans to change pension provision.
More than 750,000 were on strike – one in five of all Government employees in the UK.