The violence marked an escalation of a political crisis that has gripped Cambodia since official results from July's election gave a victory to the party of longtime Prime Minister Hun Sen, albeit with a reduced parliamentary majority. The main opposition Cambodian National Rescue Party rejected those results alleging widespread fraud.
Sunday's protests began peacefully with about 20,000 opposition activists gathering in Phnom Penh's Freedom Park for a rally addressed by CNRP leader Sam Rainsy. The protesters complained that their votes were stolen and demanded an independent investigation of the disputed election.
Clashes began later, as protesters marched in other parts of the capital and tried to remove street barricades set up by police, who responded by firing tear gas, smoke grenades and water cannons.
Chan Soveth of Cambodian Human Rights and Development Association (ADHOC) told VOA that a male demonstrator was fatally shot in the head during a confrontation on an overpass leading to Freedom Park.
"There was a clash that caused people to be injured and killed but we did not know how many were killed. We know that a young man died and we took a photo. Another injured person was shot in his stomach because armed authorities used guns to shoot, and many were arrested," he said.
The French news agency quotes military police spokesman Kheng Tito who denied that security forces used live rounds against the protesters. Another violent clash happened at a police barricade near Phnom Penh's Royal Palace. Several demonstrators were hurt.
Hundreds of Cambodian opposition activists camped in Freedom Park into the night Sunday, vowing to remain for at least several days.