* Police say no hostages being held in Luhansk
* 50 protesters leave building, others to fight on
* Talks with authorities continuing (Adds details of talks)
By Thomas Grove
LUHANSK, Ukraine, April 9 (Reuters) - Pro-Russian separatists reinforced barricades around the state security building in the eastern Ukrainian city of Luhansk on Wednesday and called on President Vladimir Putin for help after the government warned it could use force to restore order.
But protesters were also engaged in talks to ease the standoff, which Kiev has said could provide a pretext for a Russian invasion, and lawmakers from eastern Ukraine proposed an amnesty for protesters to defuse tension.
The former KGB headquarters is one of three government buildings seized this week in eastern Ukraine by protesters demanding regional referendums on independence from Kiev, like the one in Crimea that led to its annexation by Russia.
Tensions have risen in the mainly Russian-speaking east since the overthrow of Ukraine's Moscow-backed president and the installation of a new pro-European government.
"Of course we must ask Russia to take us in because I don't see an alternative," said a man dressed in camouflage who gave his name as Vasiliy and said he was the commandant of the building. "Putin help us!" he said.
Sandbags and wooden crates were piled near the entrance of the building to defend it against the police. Men with rifles could be seen through broken windows above.
Local police spokeswoman Tatyana Pogukai said protesters had found an arsenal of weapons within the building. Protesters say they have 200-300 Kalashnikov automatic rifles. She denied previous reports that hostages had been taken.
She said negotiations had been carried out overnight but the two sides had not come to an agreement.
"They won't put down their weapons until there is agreement on a referendum," she said.
Protesters in Donetsk, to the south, remain in control of the main regional authority building, but authorities have ended the occupation in the city of Kharkiv.
"A resolution to this crisis will be found within the next 48 hours," Interior Minister Arsen Avakov told reporters in the capital Kiev.
"For those who want dialogue, we propose talks and a political solution. For the minority who want conflict they will get a forceful answer from the Ukrainian authorities," he said.
Ukraine's state security service said that 50 people had left the building in Luhansk overnight. Protesters confirmed that some had left.
Activists, many in balaclavas and masks, continued to build makeshift barricades and prepared petrol bombs.
"Those who left were not ready to stay and fight," said Vasiliy, who said his "soldiers" would fight on until a referendum on independence from Kiev was held.
Ukraine's government says the actions are part of a Russian-led plan to dismember the country, a charge Moscow denies.
U.S. Secretary of State John Kerry accused Russian agents and special forces on Tuesday of stirring up separatist unrest and said Moscow could be trying to prepare for military action as it had in Crimea.
Russia denied the accusations on Wednesday and dismissed concerns over a troop buildup near the border with Ukraine in what has become the worst East-West crisis since the end of the Cold War in 1991.
"The United States and Ukraine have no reason to be worried," the Russian Foreign Ministry said. "Russia has stated many times that it is not carrying out any unusual or unplanned activity on its territory near the border with Ukraine that would be of military significance."
A lawmaker from the most popular political party in the east, the Party of Regions, on Wednesday said he planned to ask parliament to amnesty the protesters, following the success of a similar move to reduce tension in Kiev two months ago.
"The situation is so tense and complex that one stray word might cause a flare-up," said Oleksandr Yefremov. "To prevent people suffering ... we are proposing a draft law on an amnesty."
Several hundred people remained camped outside the regional administration building in Donetsk manning barricades of tyres and barbed wire as the Soviet anthem played over loudspeakers.
A member of parliament who supports the Donetsk protesters said they would remain at the building until their demands for increased independence from Kiev were met. (Additional reporting by Pavel Polityuk and Richard Balmforth; Writing by Conor Humphries and Tom Grove; Editing by Giles Elgood)