By Thomas Grove

LUHANSK, Ukraine (Reuters) - Ukraine's state security service on Tuesday said that pro-Russian separatists had placed explosives in a building they seized in the eastern city of Luhansk and were holding 60 people hostage.

But the activists, who have controlled the region's state security headquarters since storming it on Sunday, denied both charges. They said they had no explosives, but had seized an armoury full of automatic rifles.

"The criminals have rigged the building with explosives ... and are holding around 60 people, threatening them with weapons and explosives," the SBU security service said in a statement. "They are using terrorist measures."

But one of the protest leaders told Reuters that there were no explosives and that no one was being prevented from leaving the building.

"We do not need hostages to get what we want," said Anton, who declined to give his second name.

A Reuters reporter said people appeared to be moving freely in and out of the building.

The Luhansk building was one of several seized by protesters in the east of the country demanding regional referendums on independence from Kiev.

Protesters in Donetsk remain in control of the main regional authority building, but authorities ended the occupation in the city of Kharkiv.

(Reporting by Pavel Polityuk and Tom Grove; Writing by Conor Humphries; Editing by Giles Elgood)