Cambodian opposition stages second big poll 'fraud' protest
Thousands of Cambodians are protesting, for the second weekend in a row, against the ruling party's narrow election win in July.
Opposition leader Sam Rainsy said demonstrations would continue until vote irregularities were investigated.
Police used water cannons to disperse protesters who tore down barricades near the Royal Palace.
On Saturday, talks between Mr Rainsy and Prime Minister Hun Sen failed to resolve the political deadlock.
Analysts say Hun Sen, 61, faces one of the biggest tests of his 28-year rule.
The prime minister has repeatedly vowed to stay in power until he is in his seventies.
In the July vote, his People's Party (CPP) won 68 seats, compared with 55 for the opposition Cambodia National Rescue Party (CNRP).
The national election committee confirmed the results earlier this month.
But Mr Rainsy and his followers accuse Prime Minister Hun Sen of rigging the vote.
With all state institutions - including the election commission - under the thumb of Hun Sen, protests are the only way opposition supporters can challenge the result, correspondents say.'Long live democracy'
Up to 20,000 activists, workers and monks gathered in a public park in Phnom Penh under the watchful eye of riot police on Sunday.
Rural protesters were transported to the capital in large convoys of buses and trucks.
Marchers carried signs reading "Where is my vote?" and "Long live democracy".
Many also held lotus flowers symbolising peace in what organisers described as a protest based on Buddhist prayer.
The rally was planned to last three days, and Mr Rainsy warned more would follow until election irregularities were properly investigated.
"The demonstrations will continue if our demands are not met," he said.
"[People's] ballots were stolen and they are asking for justice."
The Ministry of Interior said Sunday's rally had to end by early evening, local media reports.
But many protesters defied the order and stayed in the streets, raising the prospect of a possible forced removal.
Around 200 demonstrators clashed with police near the Royal Palace, which had been sectioned off with barbed wire barricades and roadblocks.
Security forces used water cannons and smoke grenades to disperse the protesters, who threw rocks and shoes.
But most of the rally was peaceful, correspondents say.
On Saturday, Hun Sen and Mr Rainsy failed to reach an accord in talks brokered by King Norodom Sihamoni. Both sides are due to meet again on Monday.