The makeshift bomb was safely detonated by explosive experts, causing a loud bang that witnesses said shook some nearby buildings, military police spokesman Kheng Tito told AFP.
"The device would have only made a sound," Kheng Tito said.
He said police also found three M79 rockets -- tied together -- close to Democracy Park, the location of opposition protests against Prime Minister Hun Sen's recent election win.
"These devices were planted in order to cause insecurity, chaos and to intimidate the people, " Kheng Tito said.
According to final results released by the NEC on Sunday, Hun Sen's Cambodian People's Party (CPP) won 68 seats in the July polls against 55 for the opposition.
In an attempt to end the dispute, Cambodian King Norodom Sihamoni late Thursday invited Hun Sen and CNRP leader Sam Rainsy to a rare meeting in the royal palace.