The protesters had rallied in the park ahead of a three-day protest against the government of Prime Minister Hun Sen scheduled to begin Sunday, Voice of America reported.
Phnom Penh officials later banned the use of the park and marches through city streets, raising questions about the status of the protest.
On Friday, at least four people were killed when police fired on garment workers striking for higher pay. A local human rights group said more than 20 people were wounded and at least 10 people were arrested.
Opposition forces are demanding Hun Sen, who has ruled Cambodia for 28 years, step down over alleged vote fraud in a July election.