Protesters in north-western Kyrgyzstan have retaken a regional government office, after fighting off riot police armed with tear gas and stun grenades.
Hundreds of demonstrators, angered by rising fuel prices, are demanding that President Kurmanbek Bakiyev resign.
Authorities in the capital Bishkek have detained leading opposition politician Almazbek Atambayev, reports said.
The riots in the city of Talas come a day ahead of nationwide protests set to take place on Wednesday.
The unrest began early on Tuesday following the arrest of an opposition leader who was later released, reports said.
Thousands of protesters stormed the regional governor's office in Talas, took the local governor hostage, and demanded the resignation of President Bakiyev.
Another 500 surrounded the local police headquarters.
Angry crowds attacked special forces police with rocks and petrol bombs. They reportedly set fire to portraits of President Bakiyev.
Police firing teargas and rubber bullets briefly took back the building, freeing the regional governor, but a crowd of 3,000 returned to retake the building after nightfall, Reuters news agency reports.
Prime Minister Usenov vowed to crack down on the demonstrators.
"I urge the organisers of these actions to desist from what they are doing. For those that do not listen, measures will be severe," Mr Usenov said.
Late on Tuesday, security forces stormed the home of Almazbek Atambayev, the country's most popular opposition politician and former presidential candidate, in Bishkek.
PRESIDENT KURMANBEK BAKIYEV
Came to power after "Tulip Revolution" street protests in 2005
His party won every parliamentary seat in 2007 polls - which observers said did not meet international standards
Won re-election again in 2009 - but EU observers again said poll was flawed
Opposition accuses him of a media crackdown, nepotism and corruption
A fellow opposition leader said Mr Atambayev was told he was being arrested for fomenting the unrest in Talas.
Several other opposition activists were also arrested in the capital, according to local reports.
The unrest comes amid rising tensions between the opposition and the government.
In recent weeks, the authorities have clamped down on independent media, and several internet news sources are still blocked in the country, the BBC's Central Asia correspondent Rayhan Demytrie reports.
There has also been rising discontent with the role of President Bakiyev's son who was recently appointed as the head of an important government agency.
Five years ago, mass protests in Kyrgyzstan brought Mr Bakiyev to power.
He promised to fight corruption and promote democracy, but his critics say the country has become increasingly authoritarian under his rule, our correspondent says.
Last week UN Secretary-General Ban Ki-moon visited Bishkek and called on the government to do more to protect human rights.
On Tuesday, the UN said Mr Ban was "concerned" at events in Talas and urged all parties to show restraint.
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