Thames Valley police arrested them and seized placards in their possession, after telling them that they would be held under antiterrorism legislation, according to the website.
The arrested include Sokkalingam Yogalingam, Shakya Raj Fernando, Dhanushan Mahadevan and Selvaganeshan Sivasudam of the Translation Government of Tamil Eelam, it said quoting lawyer Kulasekaram Gitaratnam.
Meanwhile, the acting Sri Lankan high commissioner in London has denied having complained to police that the protestors were carrying LTTE flags.
Harrow West MP Gareth Thomas said he had inquired from Thames Valley police about the arrests.
It is prohibited in Britain to display symbols of the LTTE and other proscribed organizations.