PHNOM PENH: A senior leader of Cambodia's opposition party was stripped of his post as the vice-president of parliament in an unexpected vote Friday attended only by the ruling party, and condemned by his supporters as illegal.

The removal of Kem Sokha by a 68-0 vote was the result of maneuvering by Prime Minister Hun Sen to get rid of his most bitter critic. The vote was not on the National Assembly's agenda.

The opposition Cambodia National Rescue Party members were taken aback when the vote on a motion to remove Kem Sokha was announced. All 55 of its lawmakers in the 123-member house decided to boycott the vote in protest. Kem Sokha was not present in the house.

Describing the vote as "unlawful," CNRP spokesman Ou Chanrith said it contravenes the constitution and democratic principles.

"A vote that no one expected is unacceptable," he said, adding that Kem Sokha's post cannot be vacated unless he dies or he himself resigns.

Hun Sen has been in power for almost three decades. While Cambodia is formally democratic, his government is authoritarian and known for intimidating opponents. Relations between the government and the opposition deteriorated earlier this year after the CNRP tried to make an issue by accusing neighboring Vietnam, with whom Hun Sen's government maintains good relations, of land encroachment.

Hun Sen and his party members have long been campaigning to remove Kem Sokha, and recently two CNRP lawmakers were severely beaten by a mob calling for Kem Sokha's resignation. The two men are undergoing treatment in Bangkok and Kem Sokha had flown there to visit them.

In recent months, Hun Sen has used his public speeches to deliver what amounts to arrest orders, which are generally carried out quickly.

One such speech resulted in the arrest and subsequent trial of a CNRP senator, Hong Sok Hour, over comments he posted on Facebook criticizing a 36-year-old border agreement with Vietnam. He faces up to 17 years in prison.