By Prak Chan Thul

PHNOM PENH (Reuters) - After a month living in an office at his party's headquarters to avoid arrest, Cambodian opposition leader Kem Sokha remains defiant and said Prime Minister Hun Sen is scared of losing the 2018 election.

Kem Sokha called for "national reconciliation" talks to end a crisis that both he and the self-styled strongman, Hun Sen, have warned could nudge the Southeast Asian nation into political conflict.

Tension has risen as Sokha and other opposition members face legal charges they say are trumped up by a judiciary in thrall to Hun Sen. The prime minister was intimidating the opposition in an attempt to avoid a rerun of a 2013 election that nearly cost him the premiership, Sokha said.

"What he is scared of most is defeat in the election," Sokha told Reuters in his first interview since he holed up at the party headquarters on May 26. "His strategy is to remove the opposition party leadership, so now he is targeting me."

Sokha became the acting head of the Cambodia National Rescue Party (CNRP) when the party's leader, Sam Rainsy, was forced into exile late last year, also to avoid arrest.

Sokha is wanted for failing to appear in court for questioning over a sex scandal investigation. He faces an accusation of procuring a prostitute.

He said the CNRP would win in 2018, even though the vote would not be free or fair, because people want change after 30 years under Hun Sen and his Cambodian People Party (CPP).

Sokha sits in his office, the corner where he sleeps curtained off. His family sits in part of it, and the diplomats and other visitors that come meet in a glass room.

Hundreds of his supporters have joined him at the CNRP headquarters, cooking and mingling to pass the time.

The opposition, critics, and the European Union say Hun Sen is using the judiciary to weaken the opposition. The EU has threatened to review nearly half a billion dollars in aid to Cambodia if Hun Sen continues harassing the opposition.

Hun Sen has denied wrongdoing and defended his tactics.

"If the judiciary isn't used, it's only guns, " he told foreign diplomats last week. "And if guns are used, it would be chaos."

Sokha said Cambodia was returning to something akin to the one party rule of the 1980s, with Hun Sen in control of the judiciary, the police and the parliament.

The only solution was dialogue, Sokha said, but the CPP says criminals must pay for their crimes.

(Reporting by Prak Chan Thul; Editing by Simon Webb and Nick Macfie)