PHNOM PENH, Cambodia — Cambodia's long-serving prime minister said Wednesday that his party was willing to talk with the opposition to resolve complaints that last weekend's general election was unfair. The gesture, from a leader not usually given to compromise, represents an acknowledgement that his opponents' strong showing in the polls could threaten his grip on power.

Prime Minister Hun Sen's ruling party has claimed that it won 68 parliamentary seats to the opposition's 55 in Sunday's election according to provisional official results.

However, the opposition, which increased its number of seats from 29 in the last National Assembly, could boycott the opening of parliament, leaving the lawmaking body short of a quorum and stymieing the formation of a new government.

Hun Sen spoke at his first public appearance since Sunday's polls, striking a conciliatory tone without making any major commitments. He said his Cambodian People's Party was ready to compromise.

The prime minister said that senior officials from his party were ready to open talks with their counterparts in the opposition, and that while he felt constrained from taking part because he was in mourning for the recent death of his father, he would talk directly with opposition leader Sam Rainsy if necessary.

His offer came after Sam Rainsy's opposition Cambodia National Rescue Party escalated its challenge to the election results, claiming it had actually won a majority of assembly seats. The party did not immediately respond to Hun Sen's remarks.

The party had already called for an investigation of voting irregularities, including registration problems that could have disenfranchised more than 1 million people. There have also been accounts of people voting who were not entitled to.

Yim Sovann, a spokesman for the opposition, said that that based on reports from party workers and election observers, his party had won at least 63 of the assembly's 123 seats. Sam Rainsy made a similar claim late Tuesday to a small group of reporters.

Speaking at the construction site for a flyover bridge in Phnom Penh, the capital, Hun Sen conceded that his party had lost seats, but said it led according to provisional results.

"Whether the number of seats in parliament increases or decreases is not important for us, but the main point is working for the sake of our entire nation," said Hun Sen, whose party had 90 seats in the outgoing assembly.

He told reporters that Cambodians should settle the matter among themselves rather than rely on the assistance of other countries — a remark that could be taken as a dig against Sam Rainsy, who actively lobbies foreign governments and rights organizations for support.

Hun Sen said he had heard that the state National Election Committee — generally seen as pro-government and criticized for failing to address registration problems before the election — was inclined to support the opposition's demand to set up an independent committee to investigate election problems.

If this was true, he said, it was a positive development.

The opposition's complaints have been supported by a number of nonpartisan Cambodian and foreign groups.

U.S.-based Human Rights Watch charged Wednesday that the ruling party "appears to have been involved in electoral fraud" in the election, citing residents and ruling party officials it interviewed.

"Senior ruling party officials appear to have been involved in issuing fake election documents and fraudulently registering voters in multiple provinces," Brad Adams, the group's Asia director, said in a statement. "And people from the party seem to have been turning up in places where they clearly don't live and insisting on voting — not to mention the many other claims of fraud around the country."

Hun Sen, who has led Cambodia for 28 years, has a reputation as a tough and wily politician. His party, with the aid of a pliant judiciary, might find a legal loophole to get around an opposition boycott of parliament, or could just continue ruling as a caretaker government, but would find its legitimacy under question at home and abroad.

The opposition's victory claim is evidently part of a strategy to keep pressure on the government, especially while there is still some foreign interest in the election. The United States said Monday that it was "concerned by numerous reported irregularities in the electoral process."

"We call for a transparent and full investigation of all credible reports of irregularities," U.S. State Department spokeswoman Jen Psaki said in Washington.

The opposition party has asked voters who were denied their ballots to offer their testimony at the party headquarters in Phnom Penh. About 500 turned up Wednesday morning.

Sam Rainsy told the crowd that their collected complaints would be presented to the National Election Committee and other interested bodies.

"It is a tremendous moment in our history that despite the large-scale theft of votes, our party still won the election," he told the crowd, which chanted "Long live the CNRP."