* Three protesters jailed for up to 3 1/2 years
* Fourth defendant gets 3 1/2-year term, but suspended
* Putin stifling dissent while all eyes on Ukraine -critics
MOSCOW, Aug 18 (Reuters) - Four Russians detained during a protest against President Vladimir Putin were sentenced to prison terms on Monday after a trial critics say is part of a Kremlin campaign to stifle dissent while all eyes are on the Ukraine crisis.
Putin enjoys broad popularity at home, however, and his public standing has ridden a wave of nationalist sentiment to new heights in his Cold War-style standoff with Western powers over Russia's intervention in Ukraine.
Judge Natalia Susina ordered Ilya Gushchin jailed for 2 1/2 years and Alexander Margolin and Alexei Gaskarov to 3 1/2 years each on charges of rioting. She found a fourth defendant, Elena Kokhtareva, guilty of the same charge and gave her a more than three-year term, but suspended the sentence.
Last month, a Moscow court sentenced prominent Kremlin critic Sergei Udaltsov to 4 1/2 years in jail after accusing him of coordinating an anti-Putin demonstration on May 6, 2012, the eve of his inauguration for a third Kremlin term.
Udaltsov had been under house arrest since February 2013, as is another prominent Putin foe, blogger Alexei Navalny.
Police detained more than 400 people and dozens of officers were hurt in May 2012 after the rally turned violent.
But the defendants' supporters, some of whom shouted "Freedom!" outside the court house, say the reports of violence was exaggerated by the authorities.
Demonstrators unfurled a banner reading "Putin's Pseudo Justice is the Shame of Russia" from the roof of a building near the court house, but it was torn down minutes later by police.
Known as the "Bolotnaya case", after the square where the protest occurred, the prosecution of a dozen protesters has come to symbolise what Putin's foes regard as a crackdown to silence dissent while the world is distracted by the Ukraine crisis.
Russia annexed Ukraine's Russian-majority Crimea region in March and the Kiev government accuses Moscow of stepping up arms supplies to pro-Russian separatists fighting to stave off defeat to a Kiev offensive in the east, something the Kremlin denies.
Putin is now in his third term as president, following a spell from 2000 to 2008.
Last week one of Russia's oldest non-governmental organisations, Memorial, added the four defendants sentenced on Monday to a list of 45 it describes as political prisoners.
It said the defendants had been carrying out a non-violent exercise of the right to freedom of assembly, had been deprived of a fair trial and faced disproportionate charges.
But with the crisis in Ukraine preoccupying media at home and abroad, the case against them achieved little of the notoriety of the August 2012 Pussy Riot trial, in which members of the punk band were jailed after performing an anti-Putin song in Moscow's main cathedral.
Since starting a new six-year term in 2012, Putin has rushed a series of laws through Russia's parliament that critics say strengthen his hand to muzzle critics. The measures included legislation allowing for tougher punishment for people involved in street demonstrations and tighter controls on bloggers. (Reporting by Mikhail Antonov and Alissa de Carbonnel; Editing by Mark Heinrich)