The opposition in Kyrgyzstan says it has taken control of the capital, Bishkek, after overrunning the president's palace.
Anti-government protests have been gathering strength for weeks
Protesters confronted supporters of President Askar Akayev before flooding into government offices.
A prominent Kyrgyz opposition leader, Felix Kulov, made a televised appeal for calm after being freed from jail.
Demonstrations were stepped up after recent parliamentary elections, which the opposition said were rigged.
An unconfirmed report by Interfax news agency said Mr Akayev and his family have left Bishkek by helicopter.
Kyrgyzstan's Supreme Court has annulled February's controversial elections and recognised the former parliament as the legitimate legislature, Russia's Itar-Tass news agency quoted court chairman, Kurmanbek Osmonov, as saying.
At the palace - also the seat of government - police melted away as hundreds of protesters flooded into the compound.
Some appeared in windows, where one demonstrator was seen waving a flag as another threw out documents.
Officials were seen fleeing by the back door.
Mr Kulov, a former vice-president who was jailed for embezzlement in 2000, appealed to Mr Akayev to "meet with opposition leaders in order to peacefully and constitutionally transfer power".
Observers say 56-year-old Mr Kulov, who is also the former head of the National Security Ministry - successor of the Soviet-era KGB - is emerging as the leader of what is a fractious opposition.
The demonstration in Bishkek grew rapidly from a few hundred people in the morning to as many as 10,000 a few hours later.
Protesters chanting "Down with the Akayev clans" marched through the capital to the presidential palace, known as the White House.
HOW EVENTS UNFOLDED
1543 GMT: Kyrgyzstan's Supreme Court annuls February election
1543 GMT: Opposition leader Kurmanbek Bakiyev says Akayev has left the country
1409 GMT: Russia urges Kyrgyzstan to return to "lawful" path
1350 GMT: Opposition leader Felix Kulov makes TV appeal for Akayev to step down
1257 GMT: Kulov reported released from prison
1125 GMT: Opposition says it has taken control of state TV
0954 GMT: Protesters storm presidential compound
0830 GMT: Clashes reported in Bishkek between pro- and anti-government forces
0725 GMT: Thousands of protesters march towards presidential palace
Security forces surrounding the building repelled an initial attempt to storm the compound, but offered little resistance when the demonstrators fought back.
Clashes erupted between protesters and government supporters in nearby Ala Too Square and there were reports of some injuries.
The opposition has appeared united so far in calling for the president to resign and for new elections to be held.
But the BBC's Central Asia correspondent, Monica Whitlock, says the fact that some of the demonstrators are carrying pink banners while others carry yellow suggests there may be difficulties later finding common ground on more complicated political issues.
Opposition activists have also seized control of several regional government buildings in key towns in the south of the country.
Russia, which considers Kyrgyzstan as part of its natural sphere of influence, urged the country to "return onto a lawful path", AFP news agency quoted the Russian foreign ministry as saying.
A former Soviet republic, Kyrgyzstan is of strategic importance to Russia and the United States, both of which have military bases in the country.