Despite a CNRP campaign to boycott the election, amid threats and voter intimidation, turnout was reported at 82 percent, according to the National Election Committee.
The U.S. was joined in its condemnation of the election process by several governments, political figures and rights groups.
Cambodia's ruling party says it has won every seat in parliament, effectively turning the country a one-party state.
Opposition calls on international community to reject results of Sunday's election, which ended with a victory of the ruling party.
Prime Minister Hun Sen aiming to extend his more than three decade long rule, legitimize ballot missing his only real contender.
The demonstrations marked the last push by the opposition to lobby for a boycott of the vote following the party’s dissolution by the Cambodian People’s Party-controlled Supreme Court in November.
According to the NEC, some 80 percent of the 8 million eligible voters had cast a ballot.
The dam collapse follows heavy rains across Southeast Asia, which has led to severe flooding in several parts of Cambodia.
The Cambodian government has ordered internet service providers (ISPs) to block the websites of 15 news websites of independent outlets including Voice of America for two days before and during the country’s election.
Reuters could not open the websites of Radio Free Asia Khmer, Voice of America Khmer and Voice of Democracy on Friday.