Opposition protests since the election have spurred fears of violence, especially after Prime Minister Hun Sen responded by moving troops and armored vehicles into the capital to keep order. Hun Sen, in power for 28 years, is known for cracking down on his critics, though there has been no serious election-related violence this year.
The government, through sympathetic media, has warned that the opposition protests could turn violent, and there was speculation that the discovery of the explosives was meant to frighten people away from Sunday's rally.
Police spokesman Kirt Chantharith said it was too soon to know where the explosives came from, but denied involvement by Hun Sen's government.
"By planting explosive devices, the perpetrator had the intention of making instability and chaos in society," he said. "To participate in protesting is the legal right of the people, and government forces have the duty to provide security for them, so there is no reason for the government to do this to harm itself."
Ou Virak, president of the Cambodian Center for Human Rights, said it was highly unlikely the opposition was behind the incident because it would scare its own supporters from attending the demonstration. But he also said it wasn't in the government's interests to plant the explosives.
"But the problem is that people are not always rational," he said.
The leaders of the two parties are to meet Saturday with King Norodom Sihamoni to try to resolve their differences.