Pro-Russian protesters have stormed government buildings in three eastern Ukrainian cities.
In Donetsk, Luhansk and Kharkiv they clashed with police, hung Russian flags from the buildings and called for a referendum on independence.
Ukraine's acting president called an emergency security meeting in response.
The unrest comes amid tensions between Russia and Ukraine over the removal of pro-Moscow President Viktor Yanukovych and Russia's annexation of Crimea.
Russian President Vladimir Putin has said Moscow has the right to protect the Russian-speaking population there.
Ukraine's leaders deny the country's Russian speakers are under threat and have said they will resist any intervention in their country.
Ukrainian Acting President Olexander Turchynov cancelled a planned visit to Lithuania and called a meeting of the country's security chiefs to deal with the unrest.
In Donetsk, in what was reportedly the day's most violent protest, a large group of activists broke away from a crowd rallying in the main city square to attack and occupy the regional government seat.
After clashing with riot police and breaking through their lines to enter the building, they raised the Russian flag and hung a banner from the building. Protesters outside cheered and chanted: "Russia, Russia."
Ihor Dyomin, a spokesman for Donetsk local police, said about 1,000 people had taken part in the storming of the building.
"Around 100 people are now inside the building and are barricading the building," he added.
In Luhansk, police fired tear gas at dozens of protesters who broke into the local security service building in an attempt to force the release of 15 pro-Russian activists who were arrested earlier in the week and accused of plotting violent unrest.
Local news reports said at least two people had been injured in clashes, and TV pictures from the scene showed a riot policeman being taken away on a stretcher.
And in Kharkiv, several dozen people also entered the regional government building after breaking through police lines.
They waved Russian flags out of windows as a crowd outside cheered and chanted. Police officers reportedly refused to use force against the crowd and moved away from the government building after the pro-Russian supporters broke in.
Ukraine's Interior Minister Arsen Avakov accused President Putin and Mr Yanukovych - who was forced from office in February following months of street protests and is now living in exile in Russia - of "ordering and paying for another wave of separatist turmoil in the country's east".
In a message posted on his Facebook account, he said: "The people who have gathered are not many but they are very aggressive. The situation will be brought under control without bloodshed. But at the same time, a firm approach will be used against all who attack government buildings, law enforcement officers and other citizens."
Tensions are running high between Ukraine and Russia, with thousands of Russian soldiers still said to be deployed along the border.
The new administration in Ukraine has faced continuing opposition from Ukraine's Russian-speaking regions.
Meanwhile, Ukraine's far-right Svoboda party has reported that the body of one its activists was found on Saturday after it was dumped in the woods with signs of torture, a day after his abduction in the central village of Vygrayev.
Svoboda was one of the participants in the protests that toppled Mr Yanukovych's administration.