President Vladimir Putin threatened that US sanctions against Moscow could harm Western energy interests in Russia, which the West blames for stoking the worst confrontation since the end of the Cold War.
US Secretary of State John Kerry hit back, urging Moscow to "leave Ukraine in peace" and vowing to "defend every single inch" of NATO territory. Ukraine is not a member of the North Atlantic Treaty Organisation, but several of its neighbours are.
Police had tried to hold off an angry mob of some 1,000 demonstrators with grenades and tear gas but eventually turned over their headquarters and weapons to the rebels.
Ukraine's interim president Oleksandr Turchynov lashed out at what he called the "inaction" and in some cases "treachery" of the police services in the east.
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Lugansk is one of more than a dozen towns in the east of the country that have fallen under the control of the rebels, who do not recognise what they see as the "fascist" Western-backed authorities in Kiev.
"There are neither Russian instructors, nor special units, nor troops there," said Putin.
And he hit back at Western sanctions against Russia, unveiled on Monday, warning that foreign companies operating in the lucrative Russian energy market could suffer as a result.
"If this continues, we will of course have to think about how (foreign companies) work in the Russian Federation, including in key sectors of the Russian economy such as energy," Putin told reporters.
Putin's comments threaten the operations of some of the world's biggest energy companies in the resource-rich state -- once viewed as a reliable alternative to unstable natural gas and oil producing countries in the Middle East.