Cambodia's opposition leader on Monday rejected the results of a weekend election showing a win for the long-time ruling party, raising fears of post-poll instability and setting the stage for a new showdown with Prime Minister Hun Sen.   

The challenge by opposition leader Sam Rainsy, who returned from exile last week to campaign for his Cambodia National Rescue Party, comes despite his party's relative success in Sunday's polling, in which the opposition made its biggest gains in years.   

Provisional results from Sunday's voting showed the opposition capturing 55 of the 123 seats in the National Assembly. Prime Minister Hun Sen's Cambodian People's Party won 68 seats, or a majority of 55 percent.  

Rainsy _ who had earlier vowed mass protests if the voters' will was denied _ called for an independent investigation into allegations that as many as a million people may have been deprived of their right to vote, among other irregularities.   

He said the challenge was not a bargaining chip to get into government but instead a sign that his party was ``interested in rendering justice to the Cambodian people to ensure that the will of the Cambodian people not be distorted or reversed.''   

The rejection of the results raised the specter that Cambodia might return to a previous pattern of post-election instability that has several times led to political gridlock and turned violent.   

If the opposition party boycotts the assembly, it may be impossible for Hun Sen to legally form a government.   

Rainsy did not specifically threaten a boycott, but election experts pointed out that the constitution says a quorum of 120 assembly members is needed to open a parliamentary session, raising the possibility that an opposition boycott could leave the country without a fully functioning government.

Cambodia faced a similar situation most recently after its 2003 election, when Hun Sen's party failed to win enough seats to legally form a government on its own. The deadlock was broken only after 11 months and violence in the streets. But Hun Sen faced a divided opposition then, while his opponents this time are united.   

Other polls in recent decades have been followed by confrontations and violence.   

After his party ran second in U.N.-sponsored elections of 1993 _ the culmination of a process to end decades of civil war after the Khmer Rouge's murderous 1970s regime _ Hun Sen insisted on being named co-prime minister. He then ousted his partner in government four years later in a bloody coup.   

Recent years' elections have mostly had a peaceful aftermath because Hun Sen's party, which controls most levers of power, won decisively over a divided opposition. But the strong showing of the more-united opposition this year may embolden Rainsy and his allies.   

Hun Sen has not spoken publicly since the election. At 60, he has a reputation as a wily survivor, starting with his defection from the Khmer Rouge to Vietnam, which after invading to oust the radical regime installed him first as foreign minister and later as prime minister.   

Rainsy, 64, has long been the thorn in Hun Sen's side. He spent the Khmer Rouge years in France and served as finance minister in the government elected in 1993, but was kicked out from his party and his post for his outspoken anti-corruption stand.   

He founded his own party in 1995, and two years later Rainsy narrowly escaped being killed in a grenade attack on a rally he was leading.   

Rainsy said shortly after polls opened Sunday that his party would wait before deciding what to do about the alleged irregularities, but added that if it was clear the voters' will was being denied, ``definitely, there will be protests.''  

The opposition's challenge could be mostly bluster. Hun Sen's party dominates nearly all the state bureaucracy and the courts, which will almost certainly affirm the CPP victory. It was unclear what the opposition would do if its complaints were not sustained.   

On Monday, the U.S. called for a probe into irregularities into the election.

``We are concerned by numerous reported irregularities in the electoral process,'' State Department spokeswoman Jen Psaki said in Washington. ``We have consistently called on the Royal Government of Cambodia to address systematic flaws _ systemic flaws such as problems in the voter registry and unequal access to the media. We call for a transparent and full investigation of all credible reports of irregularities. `'   

The United States and other countries had expressed doubts before the election about its fairness, but are unlikely to pursue the point with enthusiasm. They have accepted results of past elections with much more open intimidation and violence, and will likely regard this year's results as a major step forward.  

The opposition could nonetheless cause a lot of mischief by refusing to take its seats. Hun Sen could seek to open parliament through a legal loophole, though such a move would support charges of unfairness and autocratic behavior.   

He could also simply try to wait out his opponent as head of a caretaker government. The position would be awkward, but also preserve the status quo, which leaves him in power.   

Rainsy's party and nonpartisan groups charged that the ruling party used the machinery of government and security to reward or pressure voters. They also said that voter registration procedures were badly flawed, possibly leaving more than 1 million people disenfranchised.   

The combined opposition had held just 29 seats in the last assembly. It was a precarious foothold _ they were kicked out on highly technical grounds by their ruling party colleagues just before campaigning began.   

But with many younger voters participating in this election, the opposition apparently gained seats with their support, analysts said.     

``The run-up to elections has shown the emergence of a young generation, which rather than prizing stability as their elders, conceived of the elections in terms of `change or no change,''' noted Astrid Noreuron-Nilsson, a Cambodia scholar from the University of Cambridge.   

She added that Hun Sen will need to learn to work with the opposition. ``Otherwise, there is a real possibility that a politically polarized population will raise the risk for social tension and social unrest.'' (AP)