Thousands of supporters of Cambodia's opposition have gathered in the capital Phnom Penh to demand an investigation into the July election that preliminary results indicate the ruling party narrowly won.

Despite the opposition's insistence that Saturday's protest will be peaceful, there is a pronounced security presence on the streets as Robert Carmichael reports from Phnom Penh.

Supporters of the opposition Cambodia National Rescue Party (CNRP) streamed into Phnom Penh's Freedom Park early Saturday for what has been touted as a peaceful gathering for prayer and reflection - not a demonstration.

For weeks the opposition has called for an investigation into the July 28 general election that it insists it won. Opposition leader Sam Rainsy has claimed on many occasions that there were massive voting irregularities.

However until now, the CNRP and the ruling Cambodian People's Party - the only two parties to win seats - have failed to agree on the composition of an investigation committee.

On Friday, a coalition of rights groups and election monitors said the ballot was not free and not fair, citing a litany of problems before, during and after voting day.

The official results will be announced on Sunday.