Ukraine's interim president Oleksandr Turchynov has called an emergency meeting of his security chiefs after pro-Russian demonstrators 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 Oleksandr Turchynov has called an emergency security meeting in response.
Czech President Milos Zeman says NATO forces should stand ready to get involved if Russia intervenes militarily in eastern Ukraine.
Meanwhile, a Russian soldier has shot dead a Ukrainian naval officer in eastern Crimea, Ukraine's Defence Ministry said on Monday, in one of few fatalities reported since Russia took control of the Black Sea peninsula, Reuters reports.
The ministry gave few details of the death late on Sunday in the town of Novofyodorovka. Russian forces took over Crimea in an almost bloodless operation before Moscow annexed the region.
The unrest comes amid tensions between Russia and Ukraine over the removal of pro-Moscow president Viktor Yanukovych and Russia's annexation of Crimea, the BBC reports.
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.
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, reports the BBC, they raised the Russian flag and hung a banner from the building. Protesters outside cheered and chanted: "Russia, Russia."
Donetsk police spokesperson Ihor Dyomin says about 1000 people stormed the building, with about 100 inside and barricading it.
In Lugansk, 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 say at least two people had been injured in clashes, and television pictures from the scene showed a riot policeman being taken away on a stretcher.
And in Kharkiv, the BBC says, several dozen people also entered the regional government building after breaking through police lines.