Japan sent election monitors to Cambodian elections in 1993, 1998, 2003 and 2008, but did not do so in last month's national elections.
The CPP jailed Sokha in September on treason charges and banned the CNRP over allegations of a conspiracy to overthrow Hun Sen by force.
The annual meeting of the 10-nation bloc opens with security concerns.
The tycoons named by Global Witness were logging baron Try Pheap, Cambodian People’s Party senators Ly Yong Phat, a sugar magnate, Mong Reththy, and Lao Meng Khin, one of Hun Sen’s closest private sector allies.
Nearly 7 million voters, or 82.89 percent of the electorate, turned out to vote, according to the NEC.
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.