The nation has been stuck in a political impasse since Premier Hun Sen's long-ruling Cambodian People's Party (CPP) claimed it had won last month's poll -- one of the most hotly contested votes seen in the country.
Rainsy, who returned from self-imposed exile to lead the opposition after receiving a surprise pre-election royal pardon, has claimed his Cambodia National Rescue Party (CNRP) actually won the July 28 poll.
"Tomorrow we will hold a big rally... To demand the UN comes and resolves the election problem in order to protect the victory of the Cambodian people," Rainsy told a small rally in the capital Phnom Penh.
But the result -- marking the lowest support for Hun Sen's party since 1998 -- has been rejected by the opposition, who claim widespread voting irregularities.
Rainsy has said his party won a majority of 63 seats and has vowed to prevent the CPP "stealing victory".
The official result is not expected until later this month.
Rainsy's CNRP on Saturday agreed to work with the CPP and the National Election Committee to set up a joint committee to probe alleged election fraud.
They say the international body can only act as an observer.
"We do not trust NEC at all. We will join the investigation only when the UN comes," Rainsy told reporters.
On Friday the UN said that disputes over the election must be "adjudicated fairly" but added it had not yet been asked to join any enquiry.
The United States also urged a probe into alleged misconduct but said the opposition gains marked a positive step toward democracy.