Government supporters gathered at the UNP headquarters in the suburb of Pitakotte to disrupt the meeting of the 'Force for National Harmony', a joint opposition initiative on human rights and democracy.
"My car was surrounded by this unruly mob who tried to break open my car window in order to pull me out of it," leader Ranil Wickremesinghe said.
The protesters said the UNP was harbouring the television crew of the British Channel 4.
"We can't allow the UNP to welcome Channel 4 who had acted against Sri Lanka", a protester in the group said.
Earlier, the Channel 4 crew was turned back from heading toward the northern town of Vavuniya after protesters sat on the rail track at Anuradhapura and did not allow the train to proceed further. Police intervened and the crew was sent back to Colombo.
Commenting on the incident at a public event, British Foreign Secretary William Hague said he was disturbed by the protest which prevented Channel 4 from going to the north of the country.