The government in Kyrgyzstan is struggling to retain power as deadly clashes escalated between police and thousands of protesters.
At least 17 people were killed in the capital, Bishkek, as protesters stormed government buildings and TV offices. Police reportedly fired live rounds.
President Kurmanbek Bakiyev declared a state of emergency in affected areas.
The protesters are angry at rising energy prices and accuse President Bakiyev of failing to curb corruption.
A detained opposition leader, Temir Sariyev, who was freed on Wednesday by protesters, says he is in negotiations with the president. There has been no word from the president's office and his whereabouts remain unclear.
However, one TV channel said talks were under way between opposition leaders, including Mr Sariyev, and the president's office. The opposition says the president and other government leaders must step down.
There are fears that the death toll could rise. One doctor in Bishkek, Akylbek Yeukebayev, told Reuters news agency there were "dozens of dead bodies, all with gunshot wounds".
Kyrgyzstan, a strategically important Central Asian state, houses a key US military base that supplies forces in Afghanistan. Russia also has a base there.
The US embassy in Bishkek and Russia have both expressed concern and called for restraint.
A spokesman for Ban Ki-moon said the UN secretary general was "shocked by the reported deaths and injuries that have occurred today in Kyrgyzstan. He urgently appeals for dialogue and calm to avoid further bloodshed".
The unrest broke out in the provincial town of Talas on Tuesday and spread to Bishkek and another town, Naryn, on Wednesday. All three are now under curfew.
Ilyas Amadiyarov, a student in Talas, told the BBC World Service by telephone that protesters there had broken through a line of police, scattering them. He could still hear gunfire which appeared to be coming from the security forces. His account cannot be verified independently.
The student said at least 4,000 people were involved in the protest in Talas, some of them coming in by car from surrounding villages.
Interior Minister Moldomusa Kongatiyev, who was believed to have gone to the town to calm the situation, was reportedly severely beaten.
Some reports said he had been killed by the mob, others that he was taken hostage, but there is no confirmation of his fate.
Bloodstains
In Bishkek, protesters attempted to storm the president's office but were held back by security forces, who reportedly fired live rounds into the crowd.
In addition to the 17 confirmed deaths, the minister of health said 180 people had been injured, but did not say how many were police or protesters.
Earlier, police had used tear gas and stun grenades to try to break up crowds outside an opposition headquarters but the protesters overcame the police and marched to the presidential offices in the city centre.
Police cars were overturned and set alight and officers attacked by the crowd, some of whom were armed.
Gunfire could be heard crackling through the centre of Bishkek and photographs from the city showed bloodstains on the pavement. The prosecutor's office was also set alight.
Mr Sariyev had been arrested after arriving on a flight from Moscow on Wednesday and a number of other opposition leaders were also detained.
There are reports up to 10 leaders have now been freed but these have not been confirmed.
