Colombo, Nov 14 (IANS): Sri Lanka's main opposition United National Party (UNP) is boycotting the Commonwealth leaders' summit after a pro-government group attacked the party's headquarters here Thursday, an official said.

The UNP's headquarters on the outskirts of capital Colombo was the venue for a human rights exhibition on the many people who have disappeared during the country's protracted conflict.

Dozens of supporters belonging to an extremist Buddhist organization Buddhist Power Force blockaded the headquarters resulting in an argument between those within the UNP building and the protestors.

The protestors stormed the headquarters alleging that the UNP was harbouring "extremist elements" within the building.

Police managed to bring the situation under control after several hours.

Members of the Criminal Investigation Department (CID) later arrested one person attending the exhibition on suspicion of having ties with the Tamil Tigers.

However, the UNP insists that the detained individual was a family member of one of the people abducted during the conflict.

"Due to the repressive acts by the government, the UNP has decided that we will not attend the opening ceremony of the Commonwealth Heads of Government Meeting (CHOGM) nor will we attend the sessions over the weekend," Tissa Attanayake, secretary general of UNP, told reporters.

UNP leader Ranil Wickremesinghe was expected to attend the CHOGM opening ceremony presided over by Prince Charles and Sri Lankan President Mahinda Rajapaksa.

Wickremesinghe's vehicle was attacked by pro-government protestors Wednesday as he attempted to visit the exhibition.

Meanwhile, the military and police Wednesday turned back relatives of dozens of people who disappeared during the war who attempted to enter Colombo to stage a protest demanding a credible investigation.

Sri Lanka ended a bloody three-decade conflict with Tamil Tiger rebels in 2009 but allegations of war crimes, failure to investigate thousands of disappearances and lack of media freedom continue to bedevil the regime of President Rajapaksa.