The ruling party of President Ilham Aliyev appeared to be leading in parliamentary elections here, according to early returns Sunday. But the vote was condemned as fraudulent by the leading opposition group, which said its supporters would take to the streets to protest.

Hundreds of international election monitors spread across Azerbaijan to observe the voting. Their judgment on the ballot's fairness, which is expected to be announced Monday afternoon, could have a significant impact on the opposition's next steps as well as how the results are received by foreign governments. Western nations, including the United States, had called for a fair ballot in this strategically important and energy-rich country on the Caspian Sea.

Opposition leaders and an election-monitoring group underwritten by international grants cited violations, including ballot-box stuffing and instances of people casting ballots that had been marked before the voters reached polling booths. Journalists also witnessed the abandonment of the vote count in a key district in the capital where an opposition leader appeared to be winning.

What remains unclear is whether the apparent violations are systemic and whether Western election observers will deem the vote tainted.

"It's even worse than our most pessimistic expectations," said Ali Kerimli, leader of the Popular Front of Azerbaijan, one of the opposition parties in a coalition that has challenged Aliyev's authoritarian rule.

Ruling party officials and the head of the Central Elections Commission said allegations of widespread fraud had no foundation. The party officials also claimed to have won a clear majority of seats.

"The elections were fair, and the opposition's claims are baseless," said Ali Ahmadov, a member of parliament and deputy chairman of the ruling New Azerbaijan Party.

Complete results will not be released until later Monday, at the earliest. More than 1,500 candidates ran for the 125 seats in the national parliament, the Milli Majlis. The legislature could play a key role in overseeing the billions of dollars expected to flow into the country, tucked between Russia and Iran, when a new oil pipeline comes on line next year.

The Interior Ministry warned Sunday that it would break up any illegal demonstrations. The opposition nonetheless said it was planning to hold a rally in central Baku and in other cities on Tuesday. The opposition said it would pay close attention to a report on the election from the Organization for Security and Cooperation in Europe.

The country's last nationwide election in 2003 was followed by violent clashes between police and the opposition. Western election observers said that vote was rigged.

The OSCE and other observers have criticized the conduct of this year's electoral campaign, pointing to the breakup of opposition rallies and the government's near-dominance of broadcast media.

Opposition leaders said the OSCE should condemn the irregularities in the balloting.

"If the OSCE doesn't clearly condemn the violations, it will be the death of democracy in Azerbaijan," Kerimli said in an interview Sunday night.

The government argued that the opposition was trying to disguise its weakness by raising the specter of widespread fraud.

"The campaign was successful," Aliyev said after casting his vote Sunday in a central Baku school. "Equal measures were provided for all the candidates and, in general, the whole election process was smooth and peaceful."

In at least one key constituency in Baku, however, there was chaos Sunday night. Election officials in 26 of 28 polling stations aborted the count after Kerimli appeared set to win. The officials fled with ballots, and police forced out observers, according to representatives for independent candidates and the opposition coalition.

Journalists found nearly 100 ballots marked for Kerimli abandoned in School 104, a polling place in the district. In School 278, members of the elections commission seized the ballots and locked themselves in a room as police asked observers and journalists to leave the building. Later, police formed a ring around the district's central election office.

"Police came in and ordered the count to stop," said Faig Ahmadov, an observer for an independent candidate in the district.

The chairman of the electoral district, Faramaz Abbasov, said after midnight Sunday that he had received only two signed result sheets from 28 polling stations, which prevented him from declaring a winner.

Nationwide, just under 47 percent of the country's 4.7 million registered voters cast ballots, election officials said.

"Even if only 10 percent of the oil money is spent on the people, our lives will improve," Azamat Mamishov, 46, said after casting his ballot for an independent candidate in a neighborhood on the edge of Baku. Mamishov's modest hopes reveal the deep cynicism here about the level of corruption in the country; throughout the campaign, both the ruling party and the opposition have stressed their commitment to using the coming bonanza wisely.

Some voters on Sunday defended Aliyev as working to transform the country to everyone's benefit. "The president is a good man," said Vagif Asadov, 42, who described himself as a small entrepreneur. "He will work for us."