Cambodia's main opposition party is denouncing a large demonstration in the capital they say was staged by supporters of Prime Minister Hun Sen to intimidate rivals ahead of July elections.

More than 10,000 protesters gathered in Phnom Penh on Sunday to lash out against opposition leader Kem Sokha for allegedly saying that exhibits at a Khmer Rouge-era genocide museum in Phnom Penh were faked.

Sokha denies the allegations, and his party says the campaign against him is politically motivated.

On Friday, the National Assembly approved a bill making it a crime to deny that atrocities were committed by the country's genocidal 1970s Khmer Rouge regime — a law critics allege is being used as a weapon against Hun Sen's opponents.

Opposition lawmakers were expelled from the legislature this week.