Update as of 2:37am EDT: Pro-Russian protesters who stormed the state security building in the city of Luhansk, in eastern Ukraine, on Sunday, have now seized weapons and have shut the building's entrances, news reports said Monday, citing local police.
"Unknown people who are in the building have broken into the building's arsenal and have seized weapons," Reuters reported, citing a statement released by the local police. 

Pro-Russian protesters clashed with police and stormed government buildings in three different Eastern Ukrainian cities. In Donetsk, Luhansk and Kharkiv, the demonstrators demanded a regional referendum on leaving Ukraine and joining Russia.

They waived Russian flags in the three cities and in Donetsk the protesters called for a “people’s city council” to be formed.

At least three people were injured in Luhansk, where police used tear gas to clear protesters who stormed a security building to try to break 15 pro-Russian demonstrators free. Photos from the protests show demonstrators breaking off the sign on a Ukrainian state security building.

In Donetsk, according to a police spokesman, 1,000 people took part in the storming of the regional government building. He said 100 people are occupying the building. President Viktor Yanukovych fled Donetsk, a pro-Russian stronghold in eastern Ukraine, after he was ousted from power in Kiev.

In Kharkiv, police refused to use force to break up the crowd and moved away once pro-Russian supporters took control of the regional government seat. Kharkiv is the second-largest city in Ukraine with a population of 1.5 million people, most of them pro-Russian supporters.

Kharkiv, Donetsk and Luhansk are each within 40 miles of the Russian border, where up to 40,000 Russian troops are stationed. Some say the Russians are planning to invade eastern Ukraine much like they invaded Crimea, but the Russian government has strongly denied that.

Ukrainian Interior Minister Arsen Avakov said Russian President Vladimir Putin and ousted Ukrainian President Viktor Yanukovych were behind the unrest. Avakov accused them of “ordering and paying for another wave of separatist turmoil in the country’s east.”

Ukraine’s acting president, Oleksandr Turchynov, canceled plans to go to Lithuania and called an emergency meeting with security chiefs to respond to the unrest. Ukrainian state security said they arrested 15 and confiscated a number of explosives and weapons in Luhansk on Saturday.