* 70 separatists detained in "anti-terrorist" operation
* But standoff continues in two other east Ukraine cities
* Lavrov says Kiev must consider Russian-speaking regions
* Britain fears Russia plans to disrupt Ukraine election
By Thomas Grove and Lina Kuschch
LUHANSK/DONETSK, Ukraine, April 8 (Reuters) - Ukrainian police cleared pro-Moscow protesters from a regional administration building in a lightning night-time operation, but others held out in two more eastern cities on Tuesday in what Kiev says is a Russian-led plan to dismember the country.
Shots were fired, a grenade thrown and 70 people detained as officers ended the occupation in the city of Kharkiv during an 18 minute "anti-terrorism" action, the interior ministry said.
But elsewhere in Ukraine's mainly Russian-speaking industrial heartland, activists armed with Kalashnikov rifles and protected by barbed wire barricades vowed there was no going back on their demand - a vote on returning to Moscow rule.
In the city of Luhansk, a man dressed in camouflage told a crowd outside an occupied state security building: "We want a referendum on the status of Luhansk and we want Russian returned as an official language."
"We will not let fascism pass," he shouted, leading the crowd in chants of "Russia! Russia!".
Ukraine says the seizure of public buildings in eastern regions on Sunday night is a replay of events in Crimea, the Black Sea peninsula Moscow annexed last month after a referendum staged when Russian troops were already in control.
Russian Foreign Minister Sergei Lavrov dismissed Western accusations that Moscow was destabilising Ukraine, saying the situation could improve only if Kiev took into account the interests of Russian-speaking regions.
But Britain expressed fears that Russia wanted to disrupt the run-up to presidential elections next month in Ukraine, which has been ruled by an interim government since the overthrow of Moscow-backed president Viktor Yanukovich in February.
Ukraine, which was controlled by Moscow until the Soviet Union collapsed more than two decades ago, has been in turmoil since late last year when Yanukovich rejected closer relations with the European Union and tilted the country back towards Russia. That provoked mass protests in which more than 100 people were killed by police and which drove Yanukovich from office, leading to Kiev's loss of control in Crimea.
In Kiev, Interior Minister Arsen Avakov partly pinned responsibility for the Kharkiv occupation on Russian President Vladimir Putin. "All this was inspired and financed by the Putin-Yanukovich group," he said.
An aide said police went in when the protesters failed to give themselves up and surrender their arms. Officers did not open fire, despite shooting and the grenade attack from the other side, he said. One police officer was badly wounded and some others less seriously hurt.
NO TURNING BACK
In Luhansk, a city of around 450,000 people, protesters have blocked streets leading to the state security building with barbed wire, piles of tyres, wooden crates, metal police barriers and sandbags. Protesters at the entrance to the building blocked the entrance with metal shields.
Andrei, who said he had stormed the building on Sunday but would not give his family name, said the protesters had 200-300 Kalashnikovs and some stun grenades, but there had been no shooting so far.
"I'll stay here until the end, until victory. Once you've taken up arms, there's no turning back. We will stay until the authorities agree to hold a referendum on the status of Luhansk," he said.
Andrei said he had sent his family to Russia after Yanukovich fled. "They couldn't stay here. I knew what would happen after the bandits took over, I knew it would only lead to war. And war is what we're getting," he said.
A standoff also continued in the mining centre of Donetsk, Yanukovich's home base, where a group of pro-Russian deputies inside the main regional authority building on Monday declared a separatist republic.
Unlike in Kharkiv, there was no clear sign that further police operations were imminent in the other two cities. "We hope the buildings occupied in Donetsk and Luhansk will soon be freed," acting president Oleksander Turchinov said.
Russia has warned Kiev against using force to end the occupations but authorities may anyway have decided not to give Moscow an excuse to intervene, holding back in the hope that the protests will fizzle out.
In London, Foreign Secretary William Hague said the occupations bore "all the hallmarks of a Russian strategy to destabilise Ukraine".
The West has expressed concern about what it says has been a buildup of Russian forces along the border with Ukraine. Moscow has said the troops are merely taking part in exercises but NATO Secretary-General Anders Fogh Rasmussen urged caution.
"If Russia were to intervene further in Ukraine it would be a historic mistake," he told a news conference in Paris. "It would have grave consequences for our relationship with Russia and would further isolate Russia internationally."
Lavrov denied responsibility for the trouble in Russian-speaking areas of Ukraine. "One should not seek to put the blame on someone else," he told a new conference in Moscow.
Turchinov said on Monday that Moscow was attempting to repeat "the Crimea scenario" in which pro-Russian forces seized the local parliament and bulldozed through the referendum, clearing the way for Russia to annex the peninsula.
Unlike in Crimea, where ethnic Russians form a majority, most people in the east and south are ethnically Ukrainian, although they speak Russian as a first language.
The dispute has raised fears that Russia might restrict its gas supplies to Ukraine's crippled economy. Russian producer Gazprom confirmed Ukraine had failed to pay for its March supplies but did not say whether it would take any action against Kiev.
Gazprom cut off supplies to Kiev during price disputes in the winters of 2008/2009 and 2005/2006, disrupting the flow of Russian gas to the EU that is carried via Ukraine. Gazprom said gas transit via Ukraine to Europe remained stable on Tuesday.
So far the United States and EU have imposed only mild economic sanctions over the Crimean annexation. However, First Deputy Prime Minister Igor Shuvalov said Russian companies listed on foreign stock exchanges should consider re-listing in Moscow. "This is a question of economic security for them," Interfax news agency quoted him as telling reporters.
(Additional reporting by Gabriela Baczynska, Jason Bush, Vladimir Soldatkin and Denis Pinchuk in Moscow; William James and Kylie MacLellan in London; John Irish in Paris; Writing By Richard Balmforth and David Stamp; Editing by Giles Elgood)