(Corrects orientation of demonstrators in para 5)
* Russian flag again flying over Donetsk building
* Pro-Kiev demonstration also gathering in city
* Pro-Russian demonstrators' numbers down
By Lina Kushch
DONETSK, Ukraine, March 5 (Reuters) - Pro-Moscow youths recaptured the administrative headquarters of the eastern city of Donetsk and flew the Russian flag from its roof on Wednesday, hours after Kiev's authorities managed to fly their own flag there for the first time since Saturday.
While the group of a few hundred pro-Moscow activists was recapturing the building, however, a far larger demonstration was gathering elsewhere in the city in favour of the authorities in Kiev, a sign that the tide of local opinion is turning against Moscow's allies in Ukraine's Russian-speaking heartland.
The pro-Moscow demonstrators, who once numbered over 1,000 but now appear reduced to a few hundred, first seized the regional administration building in deposed leader Viktor Yanukovich's home town on Monday. Ukrainian police persuaded them to leave on Wednesday after announcing a bomb threat in the building, and the Ukrainian flag was raised once they had left.
Their leader, a local businessman named Pavel Gubarev, calls himself the "people's governor" and has demanded that the regional police answer to his orders.
Donetsk has seen the most persistent pro-Russian demonstrations in a wave of protests that erupted across southern and eastern cities on Saturday just as President Vladimir Putin was declaring Russia's right to invade.
Kiev says the protests were arranged by Moscow to provide a justification for a wide invasion. It notes that all followed a pattern broadly similar to one carried out in Crimea, a southern peninsula where Russian troops quickly seized control.
In Crimea, the regional parliament was seized on Thursday by unidentified gunmen and a pro-Russian politician emerged saying lawmakers had voted behind closed doors to declare him the regional boss. Within 48 hours, Russian forces were in control.
On Saturday protesters converged on regional capitals across the south and east of Ukraine and tried to raise Russian flags and hold closed-door legislative sessions, with varying degrees of success. Donetsk appears to have been the last holdout where Russia's flag flew.
"We are determined to erect our people's power in the Donetsk region. And we will not retreat. We have huge support," Gubarev said in front of the building on Wednesday before mounting an hours-long tussle to reclaim it from police.
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 in person. On Wednesday morning he was shown on television telling the Kiev cabinet by video link that the leadership of the police force needed changing because of "sabotage from the side of the security forces".
"Everything is fine. Everyone is in place," Taruta said.
Regional police chief Roman Romanov announced he was stepping down for health reasons.
Ukraine says it has not used force to safeguard the building in Donetsk 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, leaving it with a Russian flag flying.
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. Some carried placards reading: "I am Russian. Don't protect me."
Both sides have also scheduled large demonstrations for later on Wednesday in other eastern cities, meaning the day could provide an important signal of public opinion. (Writing by Peter Graff; editing by Anna Willard and Giles Elgood)