The rocket shells were discovered at a public park near the historical Wat Phnom site, Military Police spokesman Kheng Tito told the official Chinese news agency Xinhua.
The explosive substance was found near a fence outside the National Assembly, he said.
"De-miners from the Cambodian Mine Action Center had removed the M-79 shells already, while the suspicious container of explosive substance was destroyed on the site," Tito said.
"No one was hurt in these incidents," he added. "The perpetrators aimed to cause social chaos."
Police launched an investigation to learn the identities of the perpetrators, Xinhua reported.
The explosives were found two days before a planned mass protest by the opposition against the July 28 election, won by Prime Minister Hun Sen's ruling party, Xinhua said.