"We want to protest the treatment meted out to us by the government in the recent days", Tissa Attanayake, the UNP general secretary said.
"We have decided to boycott the CHOGM entirely because the Government is using devious methods of harassing the UNP," he alleged.
Attanayake said the underworld was deployed to storm and attack his party headquarters both yesterday and today.
UNP leader Ranil Wickremesinghe said on Monday that he would attend the opening ceremony to be held tomorrow.
Attanayake said the government attempted to disrupt the joint opposition human rights event held at the party headquarters, including the inaugural meeting of the party's new leadership council.
Yesterday, the entry to UNP headquarters was blocked by a mob as party leader Wickremesinghe came to address a human rights meeting.
The protesters claimed that the UNP was harbouring a crew of the British Channel 4 TV who had run anti-Lanka documentaries accusing the country of war crimes.
