* Ukraine welcomes 'liberation' of building in port city
* Separatists still surround it, controlling access
By Alissa de Carbonnel
MARIUPOL, Ukraine, April 24 (Reuters) - The Ukrainian government said a town hall at Mariupol in the east of the country had been "liberated" on Thursday from pro-Russian protesters, but the situation on the ground was less clear.
Dozens of pro-Moscow demonstrators were surrounding it later in the day and controlling access to the building, which was still flying the separatist flag. Tyres and barbed-wire barricades remained in place
Police were working inside the five-storey city hall, which appeared to be otherwise empty.
Mariupol, an industrial port city of nearly half a million people, is one of a series of flashpoints across eastern Ukraine where Russian-speaking militants have occupied public buildings to press their demands for annexation by Russia.
U.S. President Barack Obama has threatened new sanctions on Moscow if it does not act fast to end the armed stand-offs. Russia accuses Ukrainian authorities of inflaming tensions by resorting to force.
In a statement, the Mariupol police said officers were still conducting investigations in the building after breaking up a fight overnight when separatists occupying the city hall were attacked inside by about 30 unidentified men armed with clubs.
Pro-Russian activists blamed Ukrainian nationalists for the attack but said the separatist movement was now back in charge.
One of those injured, a 20-year-old separatist who gave his name as Vova, said of the incident which left him with a bandage round his skull: "I woke up and people were running through the hall with masks. At first I thought it was our boys. But when I tried to get up they beat me with a metal pipe."
Irina Voropoyeva, 56, a separatist representing the self-proclaimed People's Republic of Donetsk, said: A group of people burst into the building screaming 'Ukraine! Ukraine!'
"They beat our people up," she said. But now the building was back under their control: "Everything is as it was," she said. "We, the People's Republic of Donetsk, still control the building. There was an attempted provocation but now it's over."
Ukraine's interior minister and president both issued statements early on Thursday welcoming the "liberation" of the building a week after a deal with Russia and Western powers at Geneva that is intended to defuse the crisis.
So far, there is little progress in vacating buildings or disarming militant members of the separatist movement. (Writing by Alastair Macdonald; Editing by Mark Trevelyan)