Civilians are believed to be among 60 people killed in clashes between police and rebels in Russia’s volatile North Caucasus, say Russian media reports.
Local officials say 50 gunmen were killed in the city of Nalchik, the capital of Kabardino-Balkaria province.
Five police officers have also died, and several are being hostage in a police station, a Kremlin envoy says.
Militants from nearby Chechnya say they attacked government buildings, according to a pro-rebel website.
Kabardino-Balkaria lies close to war-ravaged Chechnya.
A school and the city’s airport, as well as government buildings, were all caught up in the running gun-battles.
Regional President Arsen Kanokov told Itar-Tass news agency that a third of the 150 rebels who took part in attacks had been killed.
He also said 12 civilians had been killed.
Dmitry Kozak, President Vladimir Putin’s special envoy to the area, told Russian television that an operation was under way to free hostages from No 3 police precinct. He did not say how many hostages were being held.
A source at Nalchik’s Republican Hospital told Ekho Moskvy radio that 20 dead had been brought in, “all people in uniform”.
At least 40 injured people had been taken to the hospital, with more arriving all the time, the source added.
One unidentified security official has told Russian news agency RIA that the reason for the attack was the arrest on Wednesday of at least one radical extremist.
Fighting broke out in the Belaya Rechka area early on Thursday and spread to several parts of the city.
A local Interior Ministry source told Itar-Tass that rebels launched a “carefully planned” simultaneous attack on police stations, Russia’s federal security forces, military and drugs-control offices as well as the airport.
He said a security operation was under way across the city to track down and kill the militants. Shooting is now said to be sporadic.
“There was heavy fighting everywhere. Attacks have been repelled, there have been fatalities and wounded,” the official said, but adding that some of the attackers had been destroyed and scattered.
A school was also reportedly caught up in the fighting, but a teacher said all the pupils were evacuated. Parents searched frantically for their children in the school yard as black smoke billowed overhead.
The BBC’s Emma Simpson in Moscow says this appears to have been an all-out attack on Nalchik’s law enforcement and security services.
The pro-rebel Kavkaz Center website said it had received information from rebel sources that a unit of Chechen armed forces had entered Nalchik.
It said it was a detachment of the Kabardino-Balkaria jamaat, called Yarmuk. The use of the word jamaat indicates that it is made up of radical Islamic fighters.