Dozens of armed men seized the regional government headquarters and parliament on Ukraine's Crimea peninsula on Thursday and raised the Russian flag.
The door of the parliament was blockaded from inside by tables and chairs, according to a Reuters correspondent on the scene in Simferopol.
Interfax news agency quoted a witness as saying there were about 60 people inside and that they had many weapons. It said no one had been hurt when the buildings were seized early on Thursday.
"I heard gunfire in the night, came down and saw lots of people going in. Some then left. I'm not sure how many are still in there," a 30-year-old man who gave his name only as Roman told Reuters.
The seizure of the building was confirmed by the country's acting interior minister, Ukrainian television reported.
Crimea, the only Ukrainian region with an ethnic Russian majority, is the last bastion of opposition to the new political leadership in Kiev following Saturday's ouster of President Viktor Yanukovych.
Ukraine's new leaders have been voicing alarm over signs of separatism there.
Russian President Vladimir Putin has ignored calls by some ethnic Russians in Crimea to reclaim the territory handed to then Soviet Ukraine by Soviet Communist leader Nikita Khrushchev in 1954.
The United States says any Russian military action would be a grave mistake. Ethnic Tatars who support Ukraine's new leaders and pro-Russia separatists had confronted each other outside the regional parliament on Wednesday.