Cambodia's opposition Sam Rainsy Party has said it will ask the king to intervene after three of its MPs were stripped of parliamentary immunity.
Sam Rainsy has been an outspoken government critic
One MP has already been arrested after Thursday's ruling, and Sam Rainsy himself has left the country.
Cambodia's King Norodom Sihamoni has promised not to enter into his country's often murky political arena.
But a Sam Rainsy Party spokesman said he would send a letter to the king to ask that the MPs' immunity be restored.
"They should not have been stripped as parliamentarians when all they did was express their own opinions," Eng Chhay Eang said.
The government "have kicked out the opposition voice", he said, adding that Sam Rainsy Party MPs were planning to boycott parliament for a period of time in protest.
'Sad day for democracy'
On Thursday Cambodia's National Assembly achieved the two-thirds majority it needed to overturn rules protecting Sam Rainsy and his colleagues Cheam Channy and Chea Poch from prosecution.
1949: Born in Phnom Penh
1965: Leaves Cambodia for France
1989: Becomes Funcinpec's European representative
1993: Back in Cambodia, becomes finance minister
1994: Expelled from Funcinpec
1995: Founds the Khmer Nation Party, later renamed the Sam Rainsy Party
2003: Party polls 22% in general election
2005: Leaves the country after losing parliamentary immunity
As a result of the decision, the three men could face a series of defamation charges - which could force them to pay hefty fines or even spend time in jail.
Hours after the decision, Sam Rainsy left the country and is believed to have travelled first to Bangkok and then to Paris.
Cheam Channy was taken into custody by police on Thursday afternoon, and has reportedly been charged with starting an anti-Hun Sen militia.
Chea Poch is thought to be in hiding.
Prime Minister Hun Sen first filed a libel suit against Sam Rainsy last year, for accusing the government of plotting to kill its political rivals.
The other party in the coalition government, Prince Norodom Ranariddh's Funcinpec Party, has also begun defamation action against Sam Rainsy.
Sam Rainsy alleged that Prince Ranariddh took bribes to join forces with Hun Sen's Cambodian People's Party to form the government - a move Funcinpec denies.
A US senator said in a statement late on Thursday that Washington should take tough action against Cambodia for stripping Sam Rainsy of his immunity.
After parliament's decision, Prime Minister Hun Sen was unrepentant
Kentucky Republican Senator Mitch McConnell said the move was "outrageous and unacceptable", and urged the US State Department to respond in "a forceful and appropriate manner".
The US embassy in Phnom Penh also condemned the parliament's decision on Thursday.
"We are deeply concerned that the National Assembly has chosen to occupy itself with political activity that appears to be designed to silence the opposition," it said in a statement.
But Prince Ranariddh was unrepentant.
"I think this a Cambodian domestic issue. We have the full right to solve it according to the rule of the laws of Cambodia," he is quoted as saying by Reuters news agency on Friday.