* First major elections since Putin returned to Kremlin in May
* Kremlin opponents allege violations including multiple voting
* Five regional leader contests are among nearly 5,000 races
By Gabriela Baczynska and Maria Tsvetkova
KHIMKI/BRYANSK, Russia, Oct 14 (Reuters) - Russia held local elections from the Baltic Sea to the Chinese border on Sunday, testing President Vladimir Putin's grip nationwide and the reach of his opponents five months into his new term.
The ruling United Russia party was expected to win most of nearly 5,000 contests despite disaffection that cost it dozens of national parliament seats in a December vote and helped spark the biggest opposition protests of Putin's 12-year rule.
Opponents accuse the Kremlin of using its position to give its favoured candidates an unfair advantage, removing potential competitors from races.
The votes included ballots for regional governors in five of Russia's 83 provinces, the first since the Kremlin restored popular elections of regional chiefs, which Putin had scrapped during his 2000-2008 presidency as he tightened control.
In one example cited by Putin's critics as suspicious, a candidate who had posed a threat to the Kremlin's man withdrew last month from the governorship election in Ryazan province, southeast of Moscow.
Kremlin candidates faced a smooth ride in most other regions electing governors, but in the western Bryansk region a tougher race was expected against a Communist who was struck from the ballot by a court and reinstated days before the vote.
One medical worker in Bryansk accused bosses at her clinic of threatening senior staff with dismissal if they did not vote for the incumbent at multiple polling stations and record the evidence on their mobile phone cameras.
"I refused, of course," said Maria Makarova, 55, who said she voted for the Communist.
Election officials in the region could not immediately be reached for comment.
Among other closely watched votes were the only two contested by prominent leaders of anti-Putin opposition protests: activist and environmental campaigner Yevgeniya Chirikova and liberal politician Vladimir Ryzhkov.
Chirikova cried foul in the Moscow suburb of Khimki, saying hundreds of new residents had been registered in one apartment block alone shortly before the election and accusing officials at some polling places of hiding voter lists from observers.
"New residents were registered early this morning, somehow they received local registration overnight," she said at a polling station. Opinion polls gave her little chance against the United Russia-backed candidate for mayor, Oleg Shakhov.
The regional election commission chief dismissed Chirikova's allegations and Shakhov said she should "learn to lose gracefully," Interfax news agency reported.
In the Altai region capital, Barnaul, where Ryzhkov was standing, Kremlin opponents said they suspected many people were voting at multiple polling places and that voter rolls had been illegally inflated to ensure more support for United Russia.
Central Election Commission head Vladimir Churov ordered officials to look into the accusations.
Demonstrations against Putin, which drew as many as 100,000 protesters into Moscow's streets at their height, were sparked by suspicions of fraud in favour of United Russia in the December parliamentary election.
But while the protests exposed dismay among the middle class in Moscow over Putin's return to the presidency after four years as prime minister, the protest movement made few inroads deep in the provinces.
In response to the protests, the Kremlin eased restrictions that had made it very hard for political parties to officially register and contest elections, a change that means more parties were running on Sunday.
But opposition groups say they were kept off many ballots by electoral commissions that often cite minor procedural issues.
More than half the would-be candidates from the liberal People's Freedom Party were kept off or removed from ballots in first-past-the-post races nationwide, the independent vote monitoring group Golos said.