HONG KONG — Military police officers on Friday fired on protesters demanding higher wages for Cambodian garment workers, killing at least three people, officials said, as protests against the decades-old rule of Prime Minister Hun Sen entered a volatile new phase.

The garment workers are demanding a doubling of their monthly wages, and they have been at the forefront of growing protests against Mr. Hun Sen’s authoritarian government. On Sunday, tens of thousands of people rallied to demand that he step down.

But Friday’s violence south of Phnom Penh, the capital, was a sharp escalation in the unrest. Protesters resisted police efforts to break up the demonstrations, and some threw homemade explosives, setting fire to vehicles, and pelted officers with rocks and other projectiles. As the street battles raged, the police fired live ammunition and smoke canisters to try to quell the disturbances.

Another large opposition rally is planned for Sunday, again intended to turn out tens of thousands of people to force an end to Mr. Hun Sen’s rule. In July, Mr. Hun Sen’s party claimed victory in disputed elections that the opposition and many independent monitoring organizations said were deeply flawed.