(Adds new governor quotes, detail)
By Lina Kushch
DONETSK, Ukraine, March 5 (Reuters) - Ukraine raised its flag on Wednesday over the government headquarters in Donetsk where a Russian flag had stood for five days, witnesses said, an important signal of shifting control in the Russian-speaking east.
Police said they were evacuating the building, which has been occupied since Monday by pro-Russian demonstrators. A police statement said the evacuation began after reports that the building was booby-trapped with explosives.
Donetsk, home city of deposed president Viktor Yanukovich, has seen the most persistent pro-Russian demonstrations since protests erupted in eastern and southern cities on Saturday just as Russian President Vladimir Putin was declaring his right to invade.
Russian forces already have control of the southern Crimea region but have not entered other parts of Ukraine. The main concern of NATO officials meeting Russian counterparts in Brussels on Wednesday is Putin's threat to launch a wider invasion to protect Russians across Ukraine's east and south.
The pro-Moscow activists in Donetsk, led by a local businessman named Pavel Gubarev who declared himself "people's governor", have been holed up in the regional administration building. They have demanded relations with Kiev be severed and control over the police and security forces be placed in Gubarev's hands.
Kiev accuses Moscow of organising the marches to create an excuse for military intervention, busing Russians across the border to protest. Some of Gubarev's followers are clearly outsiders who speak Russian without the distinctive local accent, although others appear to be from the area.
"A message was received at 8:30 (0630 GMT) from an unknown person that there is a grenade in the regional council building," said Olga Pochkalova, police spokeswoman. "An investigative-operational team is checking and people are being led out of the building."
Witnesses said the Russian flag on the roof of the 11-storey building was replaced with a Ukrainian flag, but another Russian flag was still flying from a flagpole in front.
Pro-Moscow protesters raised Russian flags at government buildings in several eastern and southern cities on Saturday hours before Russia's parliament voted to give Putin the authority to invade.
Russian troops had already seized control of the isolated Crimea peninsula in the Black Sea even before Putin declared the right to invade, and the demonstrations in eastern and southern cities led to worry about a much wider operation.
Ukraine says it has not moved in to reclaim the building in Donetsk by force or cracked down on pro-Russian demonstrators elsewhere, to avoid violence that might provoke a Russian military response. Authorities have also acknowledged questions about the loyalty of some security forces.
Most Ukrainians in the east and south of the country speak Russian as a native language. Many people are deeply suspicious of the new government in Kiev and some have supported the pro-Russian demonstrators.
However, the protests have also produced a backlash, especially after scores of people were injured in the city of Kharkiv by demonstrators wielding axe handles and chains who stormed and trashed the government building on Saturday.
On Tuesday evening around 1,500 demonstrators marched in Donetsk waving Ukrainian flags and opposing Russian military intervention, the first time that pro-Kiev protesters outnumbered pro-Russian demonstrators in the city.
Both sides have scheduled large demonstrations for later on Wednesday in Donetsk and several other eastern cities, meaning the day could provide an important signal of public opinion.
Kiev's new government has named one of Ukraine's richest men, metal baron Sergei Taruta, as Donetsk regional governor, a sign that powerful oligarchs, many of whom once supported Yanukovich, are now behind the new authorities.
Taruta has yet to appear at the Donetsk government headquarters to take up the job. On Wednesday morning he was shown on television speaking by video link to a meeting of the Kiev cabinet.
"Everything is fine. Everyone is in place," Taruta said. "One bit of help that I need to do my job - a rapid response to personnel appointments, because there is sabotage from the side of the security forces, and therefore I need assistance." (Writing by Peter Graff; editing by Anna Willard)