OSH, Kyrgyzstan — Thousands of protesters, some armed with clubs and Molotov cocktails, overran Kyrgyzstan’s second-largest city yesterday, forcing police to flee as the government lost control of the impoverished southern region of the former Soviet republic.

Demonstrators in Osh burned and stomped on portraits of President Akayev and seized control of the airport.The army did not intervene despite the chaos. No casualties were reported.

The opposition occupied government buildings in five cities and towns across southern Kyrgyzstan, Interior Ministry spokesman Nurdin Jangarayev said. The capital, Bishkek, remained calm.

“Power in Osh has been taken over by people!” opposition member Anvar Artykov told the crowd.”I congratulate you on our victory and urge you to maintain order.”

The protests, involving more than 17,000 people in the affected cities, won the first concession from Mr. Akayev — an investigation into allegations of widespread vote-rigging in two rounds of parliamentary elections since February 27. The allegations, backed by European observers, have led to demands for Mr. Akayev’s resignation and to weeks of protests.

Although Central Asia is the last bastion of post-Soviet dictators, Mr. Akayev was regarded as the region’s most reform-minded leader. In recent years he has cracked down, and his reputation was tarnished in 2002 after police killed six demonstrators protesting the arrest of an opposition lawmaker.

An Akayev aide, Abdil Seghizbayev, said security forces would not act against the protesters and said peace talks would be possible only after order is restored. “Neither the authorities nor opposition leaders can control the crowd right now,” he said.

The demonstrations make Kyrgyzstan the next possible candidate for a revolution within the former Soviet bloc. However, the uprisings that swept Georgia and Ukraine in the past two years were peaceful — and Kyrgyzstan lacks a powerful opposition leader able to take over.

Kyrgyzstan’s opposition parties long have been fractured along regional lines running parallel to the mountain range that divides the country. The south is poorer and densely populated, while the north enjoys most of the power and resources.

The opposition is convinced that it is being shut out of political life in this mostly Muslim nation of 5 million people. Although Islamic militants have conducted raids in Kyrgyzstan in previous years, religion does not appear to be playing a role in the latest protests.

Kyrgyzstan, which borders China, Uzbekistan,Tajikistan, and Kazakhstan, is an energy-rich region of considerable interest to America and Russia, which are vying for influence in the area.

Southern Kyrgyzstan has been the scene of a series of incursions in recent years linked to the Islamic Movement of Uzbekistan, a group that fought alongside the Taliban against the American-backed government in Afghanistan. American troops and other anti-terrorist coalition forces are based at the Manas airport near Bishkek for air operations in Afghanistan.

Russia condemned Monday’s protests. “Extremist forces must not be allowed to use political instability to create a threat to the democratic foundations of Kyrgyz statehood,” the Russian Foreign Ministry said.

Moscow also rebuked the Organization for Security and Cooperation in Europe for its critical evaluation of the elections, urging it to “be more responsible in forming its conclusions to prevent destructive elements from using these assessments to justify their lawless actions.”

U.N. Secretary-General Annan voiced concern about the developments in Kyrgyzstan.”The secretary-general is opposed to the use of violence and intimidation to resolve electoral and political disputes, and calls on all parties to apply restraint,” Fred Eckhard said. “Dialogue is the only viable means for addressing the current tensions.”

The opposition has charged that Mr. Akayev, 60, who is prohibited from seeking another term, planned to manipulate the parliamentary vote to gain a compliant legislature that would amend the constitution to allow him a third term. Mr. Akayev, who has been president for 15 years, has denied wanting another term.

Roza Otunbayeva, an opposition leader and former foreign minister, on Monday ruled out any talks with Mr. Akayev. “We have one aim only: to oust this government,” she said.

But another opposition leader, Kurmanbek Bakiyev, said talks would be possible if Mr. Akayev attends them.

In Osh, many police, security forces, and local officials fled the demonstrators, some of whom shouted: “Akayev, Go!” The protesters seized the governor’s office, regional police, and security stations. About 100 police deployed near the governor’s office melted away and one who wasn’t fast enough was beaten by protesters.

About 100 protesters later took control of Osh airport, where they met no resistance, police said.Security officers sat on their packs at the airport in the face of protesters, awaiting evacuation.

“This is a new day in our history,” said Omurbek Tekebayev, an opposition official in Osh.

About 15,000 people demonstrated peacefully in Jalal-Abad on Monday, a local government spokesman said, and the Interior Ministry said hundreds more were rallying in at least two other towns.