Google blocked new downloads of a Russian opposition voting app under "extraordinary duress," sources close to the matter said Friday, as international outrage and concern built over the decision.

The company faced public threats from the Russian government and private threats of serious criminal charges and incarceration of local staff, the source added, similar to pressure that hit Apple before it pulled the app.

Allies of jailed critic Alexei Navalny accused the tech giants of "censorship" after they said a "Smart Voting" app advising supporters on how to vote out Kremlin allies had been removed from the companies' app stores.

Earlier Friday a source close to Apple's decision told AFP the iPhone maker had relented after authorities made arrest threats against its workers in Russia. 

Polls opened across the vast country on Friday after a year that saw a sweeping crackdown on President Vladimir Putin's opponents, with Russians given the option of voting online. 

The election for seats in the lower house State Duma, which runs until Sunday, comes after a year that has seen Navalny jailed, many of his allies arrested and his organizations banned.