Kyrgyzstan's parliament has appointed an interim leader after President Askar Akayev was toppled in a rebellion.
Anti-government protests had been gathering strength for weeks
Opposition MP Ishenbai Kadyrbekov was named acting president, hours after demonstrators overran the government headquarters in the capital, Bishkek.
Unconfirmed reports say Mr Akayev has fled the former Soviet republic along with his family.
As night fell looting broke out, with gangs of youths ransacking stores and setting fire to buildings in Bishkek.
The United States said it was keen to see events in Kyrgyzstan move towards a democratic process, while Russia accused the Kyrgyz opposition of causing chaos.
The Kyrgyz government collapsed following weeks of demonstrations against parliamentary elections in February, which the opposition said were rigged.
The country's Supreme Court annulled the results and recognised the outgoing parliament as the legitimate legislature.
The parliament, which was due to be replaced by the winners of the recent election, reconvened in emergency session late Thursday to appoint a new leadership.
Ishenbai Kadyrbekov, named interim president, is a former construction minister, and is described by analysts as powerful and tough.
A prominent opposition leader, Felix Kulov, who was released from jail by his supporters earlier in the day, was appointed head of the country's security ministries.
Another opposition leader, Kurmanbek Bakiyev, was named interim prime minister.
But it is not clear whether the appointments carry any legal weight.
The Associated Press news agency quoted one deputy, Temir Sariyev, as saying "nobody knows what is legitimate right now".
Foreign observers have expressed concern about possible divisions in the forces that have taken over in Kyrgyzstan.
US Secretary of State Condoleezza Rice said it was too soon to know where events in Kyrgyzstan were leading.
HOW EVENTS UNFOLDED
1820 GMT: Ishinbai Kadyrbekov appointed interim leader
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
"This is a process that's just beginning. We know where we want to go," she said.
Russia was less restrained in its reaction.
"I think that the so-called opposition, which has not controlled anything for a long time, should have the brains to find enough strength to calm down and bring the situation to the plane of political dialogue and not a dialogue of screams, shattering windows, destroying buildings and freeing prisons of criminals," said Defence Minister Sergei Ivanov.
Kyrgyzstan is of strategic importance to Russia and the United States, both of which have military bases in the country.
The demonstration which ousted the government 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.
Security forces melted away as hundreds of protesters stormed the building, smashing windows and throwing out furniture and documents.
Activists also seized control of several regional government buildings in key towns in the south of the country.
Despite appeals for calm, jubilation turned to violence as gangs of young men raided shops and set buildings alight.
In a large store on the main street, Beta, looters carted out everything from mattresses, coat hangers and mirrors, to crates of food, juice and cookies, the Associated Press news agency reported.
"There are looters and there are no police to stop them," Mr Kulov told Reuters news agency.
"The law enforcement bodies are completely demoralised," he said.