Georgia's President Mikhail Saakashvili has declared a 15-day, nationwide state of emergency after police broke up a sixth day of opposition protests.
Demonstrations have been banned, only state television can broadcast news.
In a TV address, Prime Minister Zurab Nogaideli said a coup attempt had been made and Mr Saakashvili said "Russian special services" were behind unrest.
The president has rejected the protesters' accusations of corruption and says he will not quit.
The prime minister initially announced a state of emergency in the capital Tbilisi for 48 hours, but that was later extended to the whole country for 15 days.
Earlier in the day, police had used water cannon, tear gas and batons to break up opposition protests outside parliament.
Officials said 500 people had sought medical treatment, most to be treated for the effects of tear gas.
The White House said it was "concerned" by the unrest in Georgia, the AFP news agency reported.
"We are concerned about the reports from Georgia. We urge that any protests be peaceful and that both sides refrain from violence," US national security council spokesman Gordon Johndroe said.
Speaking before he declared a state of emergency, Mr Saakashvili said his country faced "a very serious threat of unrest".
"High ranking officials in Russian special services are behind this," he said, adding that he had evidence.
He said several Russian diplomats would be expelled from Georgia for engaging in "espionage".
Earlier he had recalled Georgia's ambassador to Moscow for "consultations".
Moscow has called his allegations irresponsible and said any expulsion of Russian diplomats would be followed by an "adequate response".
The foreign ministry said the remarks were "anti-Russian hysteria".
Mr Saakashvili has rejected the allegations of corruption
Police used tear gas and water cannon after several thousand protesters tried to occupy Rustaveli Avenue - Tbilisi's main thoroughfare.
Imedi TV, which has broadcast statements by opposition leaders, stopped broadcasting on Wednesday, after riot police entered its premises.
The protesters say the police response demonstrates Mr Saakashvili's authoritarian tendencies but police said they had to unblock the capital's main road.
Opposition supporters have been gathering outside parliament every day since Friday, when 50,000 people attended the largest street protest seen since the 2003 "Rose Revolution" that brought pro-Western Mr Saakashvili to power.
The protesters accuse him of corruption and of not doing enough to tackle poverty.
They are calling for the president's resignation and a fresh election.
Many of the protesters back the president's former ally, Irakli Okruashvili, who was arrested last month shortly after he said Mr Saakashvili had plotted to kill a top businessman.
He was later released on a multimillion-dollar bail and went to Germany. The government says Mr Okruashvili's accusations are "false and baseless".