One witness said police fired the tear gas and water cannons from two fire trucks at a group of about 500 protesters.
"We fired tear gas and water cannon to disperse them from the area," National Military Police spokesman Kheng Tito told Chinese news agency Xinhua.
The protest was organized by opposition Cambodia National Rescue Party against the ruling Cambodian People's Party, which won 68 parliamentary seats in the July 28 election. The group also protests the re-election of Prime Minister Hun Sen, who has been in power or 28 years.
Protest leader Sam Rainsy told a group of about 20,000 supporters that the CNRP would boycott the opening session of parliament Sept. 23 if allegations of election irregularities weren't investigated.