Police have fired tear gas and water cannon to disperse thousands of demonstrators in Cambodia's capital, Phnom Penh, demanding an independent investigation into a July election they say was rigged.
Clashes broke out twice on Sunday between separate groups of protesters hurling rocks at hundreds of riot police who used baton charges after supporters of the opposition Cambodia National Rescue Party (CNRP) tried to remove razor-wire barricades.
Witnesses told AFP agency that at least one person had been killed and several others wounded in the clashes.
Military police spokesman Kheng Tito said he was unable to confirm the death and said that security forces had not fired live ammunition.
The latest incidents of violence and CNRP's refusal to heed instruction not to march beyond the protest site have taken a six-week standoff representing one of the biggest tests of Prime Minister Hun Sen's three-decade grip on power to a new and precarious level.
Carrying banners reading "my vote, my nation" and "where is my vote?" - in a reference to the alleged poll fraud which saw the Cambodian's People's Party (CPP) win 68 seats compared to the 55 alloted to the CNRP - many protesters carried backpacks apparently in preparation for a long stay.
By nightfall, at least 1,000 protesters were camped out in the rain on straw mats and under makeshift tents, despite
government orders to leave Freedom Park, the only venue in Cambodia where rallies are permitted.
The clashes with riot police, known for cracking down hard on dissent, added to political tension not seen for years,
heightened by the discovery of a bomb and some grenades around the city two days earlier.
We cannot accept the fake results
"We cannot accept the fake results," Kem Sokha, deputy head of the CNRP, told the rally.
"I will not betray the will of the voters," he said, adding protests will run until Tuesday and perhaps longer. Kem Sokha said opposition legislators will not attend the September 23 opening of the parliament.
Sunday's protest comes a day after a rare meeting between opposition party head Sam Rainsy and Hun Sen in an effort to break the paralysis gripping the kingdom.
Hosted by Kind Norodom Sihamoni, the meeting made little progress towards ending the political deadlock that has engulfed the nation since the July polls, which returned the Cambodian People's Party to power despite allegations of widespread vote-rigging.
The march comes after heightned tension by Friday's discovery of a home-made bomb near parliament and hand grenades close to Freedom Park.
Thus far efforts to challenge the electoral outcome have failed to overturn the CPP victory. Hun Sen, 61, has been in power for 28 years and has vowed to rule until he is 74.