06:44 a.m. Aug 23, 1998 Eastern

By Robert Birsel

PHNOM PENH (Reuters) - About 10,000 Cambodians defied a government ban and joined a demonstration on Sunday demanding a thorough investigation into opposition allegations of fraud in last month's elections.

The protest, spearheaded by opposition politician Sam Rainsy and senior officials from deposed co-premier Prince Norodom Ranariddh's FUNCINPEC party, was noisy but peaceful.

Government leader Hun Sen and his Cambodian People's Party (CPP) won the July 26 vote but Ranariddh, whose party came second and Sam Rainsy, whose party came third, have rejected the result alleging widespread fraud and and intimidation by the CPP.

``We will not accept the results of an election that was fixed, and we will not be ruled by a regime that wins through a fixed election,'' the two opposition leaders said in a joint statement.

The government on Friday refused to grant permission for the protest, saying there was not enough time to arrange security. Authorities said it could be held on Monday instead.

There were also fears of trouble following a Thursday night grenade and gun attack outside the interior ministry in which one man was killed when Sam Rainsy and a group of supporters were inside as part of their protest campaign against the polls.

Sam Rainsy blamed the attack on government supporters and said it was an attempt on his life. He also said it was aimed at finishing off opposition objections to the election and scaring people off attending the Sunday protest.

Authorities blamed unidentified ``terrorists'' for the attack and vowed to catch them but no arrests have yet been made.

In response to fears of trouble, the organizers of Sunday's protest canceled a march from Phnom Penh's main sports stadium to the National Assembly.

Inside the stadium, the crowd cheered speakers and chanted support and then snaked their way through the streets around the building as police looked on.

The demonstrators carried banners reading ``The ruling party stole the election'' and ``Stop the election cover up, recount the votes.''

Opposition officials said 28 truck-loads of their supporters were stopped on the outskirts of the city early on Sunday and police used a water cannon to disperse one group trying to march through a police barricade.

Ranariddh is on a private visit to Bangkok and did not attend the protest.

Hun Sen has offered to bring the opposition parties into a coalition but the two leaders have threatened to boycott the new parliament, blocking formation of a government, unless their complaints are properly dealt with.

The opposition has also complained that the bodies responsible for administering the election and adjudicating disputes are biased in favor of Hun Sen's party.

A U.S. group which observed the vote lent weight to the opposition complaints on Saturday, saying the National Election Committee (NEC) and the country's top legal body, the Constitutional Council, lacked credibility.

The NEC rejected all opposition complaints earlier this month. The opposition took its complaints to the Constitutional Council which has already said it will not consider many of the opposition grievances.

Opposition officials said they would hold another protest on Monday and march to the National Assembly as originally planned.