PHNOM PENH (Reuters) - A Cambodian opposition leader has been holed up in his party’s headquarters for seven days to avoid arrest on charges he says are trumped-up as Prime Minister Hun Sen looks to neutralize his opponents ahead of a 2018 election.

Kem Sokha leader of the Cambodia National Rescue Party (CNRP), arrives to visit fellow party members at the Prey Sar prison on the outskirts of Phnom Penh February 11, 2016. REUTERS/Samrang Pring

About 100 supporters of the politician, Kem Sokha, have gathered at the headquarters of the Cambodia National Rescue Party (CNRP) as tension runs high in the capital after police visited the building last week in a bid to arrest him.

Kem Sokha faces an accusation of procuring a prostitute. The case has dominated politics for weeks.

The opposition says Hun Sen is using the judiciary to weaken it and avoid a repeat of a close-run 2013 election that nearly cost him the premiership.

CNRP leader Sam Rainsy, a former finance minister, lives in self-imposed exile to avoid arrest after authorities last year reactivated an old defamation case for which he had received a pardon.

The European Union called this week for a halt to “judicial harassment”. The United Nations also voiced alarm at the tension between Hun Sen’s party and the opposition, in particular the arrest or attempted arrest of politicians.

U.N. spokesman Stephane Dujarric expressed concern again on Thursday when reporters asked him about Kem Sokha’s situation.

“The Secretary-General (Ban Ki-moon) remains concerned about reports of widespread intimidation, harassment and arrests of members of civil society organizations, the media, members of the opposition, and the National Election Committee,” he said.

Dujarric added that Ban urged “all political parties to de-escalate tensions.”

The government says criminals should face punishment and denies the charges are political.

Supporters are prepared for a long stay. They took delivery of donated food and water on Thursday and erected a shelter in front of the headquarters.

“He will stay here forever,” said CNRP lawmaker Eng Chhay Eang.

Kem Sokha would come out only if there was a solution with the veteran prime minister’s ruling Cambodian People’s Party (CPP), he said.

“We are here to protect him, no matter what happens, we are ready to die,” said Kem Sarun, 51, a farmer who travels 70 km (40 miles) a day to Phnom Penh for the vigil.

The party and its trade union allies have threatened to stage protests if Kem Sokha is arrested.

Hun Sen’s party said there would be no compromise and politicians must be held accountable.

“This can’t stop the enforcement of law, whether it is sooner or later, it will happen,” CPP lawmaker Sok Eysan said.

The accusations against Kem Sokha stem from a secretly recorded telephone conversation between him and a woman with whom the government says he was having an illicit affair.

Prostitution was made illegal in Cambodia in 2008, though it is widespread.

Kem Sokha has neither confirmed nor denied an affair with the woman and has dismissed the action against him as politically motivated.