KYIV -- Ukraine's acting president, Oleksandr Turchynov, has cancelled a trip to Lithuania scheduled for April 7 to deal with pro-Russia protests in eastern Ukraine.
Turchynov also held an emergency meeting with the heads of law enforcement agencies after protesters seized government buildings in the eastern Ukrainian cities of Donetsk and a state security office in Luhansk on April 6.
Pro-Russia protesters also stormed the regional government building in Kharkiv later on the same day.
In Donetsk, a group of protesters barricaded themselves inside the regional administration building.
They threatened to set up a "people's council" and demanded that a referendum like the one organized in Crimea be held in the Donetsk region over whether it should join Russia.
Earlier in the day, some 2,000 protesters had rallied outside the regional administration building before a group of protesters broke into the facility and raised the Russian flag.
"I came to support my people," Lyubov, a female protester, told the Reuters news agency. "We are against that gang [in Kyiv], we are against fascism. I want Customs Union [with Russia]. Kyiv is not listening to us, the European Union is not listening to us. I would be happy for talks with the EU -- let them come to Donetsk, to Donbas, and listen to us. No, they are only listening to those in Kyiv instead."
Reports from the scene said police did not take action against the protesters.
WATCH: Protesters Seize The SBU's Headquarters In Luhansk
Ukraine's independent UNIAN news agency says a female protester and a law enforcement officer were hurt during the storming of the security service's headquarters in Luhansk.
Pro-Russia rallies were also held in Kharkiv on April 6, yet another eastern city. As was the case in Donetsk, protesters called for an independence referendum like the one that led to the annexation of Ukraine's Crimea by Russia last month.
Later that night, several dozen protesters broke through police lines and entered the city's regional administration building. Police did not use force against the protesters, who raised a Russian flag over the building.
A rally against separatist tendencies and in support of preserving Ukraine's unity was also held in Kharkiv earlier on April 6.
Ukrainian media reported scuffles between pro-Russia demonstrators and a group of pro-European protesters, with police trying to keep the two groups apart.
Ukraine's Interior Minister Arsen Avakov has accused Russian President Vladimir Putin and Ukraine's ousted President Viktor Yanukovych of "ordering and paying for the separatist turmoil" in the east of the country.
The president of the Czech Republic has suggested the West should take strong measures if Russia tries to annex the eastern part of Ukraine.
Milos Zeman told Czech public radio on April 6 that such options should possibly include sending NATO forces to Ukraine.
Ukraine's mostly Russian-speaking southern and eastern regions have been hit by protests following the ouster of Moscow-backed President Viktor Yanukovych in February.
The pro-Russia protests come with Russian troops massed near Ukraine's eastern border.
On April 5, Ukraine's SBU security service said it had detained 15 people on suspicion of plotting to overthrow authorities in the Luhansk region.
In a statement, the SBU said the group "planned to carry out an armed seizure of power on April 10 in the Luhansk region through the intimidation of the peaceful population and the use of weapons and explosives."
It said a large cache of weapons was seized. No names or additional details about the alleged plot were provided.
The announcement came after Russia’s Federal Security Service (FSB) said earlier this week that it had detained 25 Ukrainians on suspicion of planning attacks in Russia.
Also on April 5, Ukrainian Prime Minister Arseniy Yatsenyuk accused Russia of "economic aggression" following recent price hikes for natural-gas supplies.
Russia twice this last week raised the price of gas for Kyiv, taking the cost for Ukraine from $285.5 per 1,000 cubic meters at the start of last week to $485.5 by the end of the week.
With reporting by AFP, UNIAN, and ITAR-TASS