At least 10,000 Cambodians have protested in the capital, Phnom Penh, expressing their anger at an opposition leader who allegedly described a notorious Khmer Rouge prison as a Vietnamese invention.

The rally on Sunday came two days after parliament passed a law banning the denial of atrocities committed by the hardline communist regime, a move that the opposition claimed was politically motivated before elections in July.

In a recording posted on a government website last month, the deputy head of the Cambodia National Rescue Party (CNRP), Kem Sokha, purportedly said that Tuol Sleng prison in Phnom Penh was staged by Vietnamese soldiers who ousted the Khmer Rouge in 1979.

The CNRP has said his remarks were doctored to cause "political trouble" before the July 28 elections, when Prime Minister Hun Sen is seeking to extend his nearly three decades in power.

Survivors from Tuol Sleng, also known as S-21, urged Kem Sokha to apologise as protesters gathered in a park in Phnom Penh before marching to the headquarters of the CNRP.

"I won't allow anyone to distort history while I'm alive. We demand that Kem Sokha lights incense sticks and apologises before the souls of the dead," said 83-year-old survivor Chum Mey, who led the protest.

Local media reported thousands more people protested in provinces across the country.

Tortured and executed

Around 15,000 men, women and children from Tuol Sleng were tortured and executed during the "Killing Fields" era.

The former head of the prison, Kaing Guek Eav, better known as Duch, was sentenced to life in prison last year.

Protesters carried banners reading: "Kem Sokha is the first person who dares to insult the souls of all victims from Pol Pot's regime" and "Kem Sokha is more cowardly than Duch".

"I feel very hurt and I am angry with what he said," said Nov Sorn, 61, who lost her husband, father and a brother under the communist regime, which oversaw the deaths of an estimated two million people.

Pro-government media recently publicised comments allegedly made by Sokha that exhibits at Phnom Penh's famous Tuol Sleng genocide museum were faked.

Yim Sovann, a spokesman for Sokha's party, said the rally Sunday was "orchestrated" by ruling party supporters.

"The protest was orchestrated by the Cambodian People's Party," he told the AFP news agency, alleging that people were paid to attend.

Under the new law, which was approved on Friday by a parliament controlled by legislators from Hun Sen's party, anyone denying Khmer Rouge atrocities risks a prison sentence of up to two years.

Led by Pol Pot, who died in 1998, the Khmer Rouge from 1975-79 wiped out nearly a quarter of Cambodia's population through starvation, overwork or execution in a bid to create an agrarian utopia.