Pro-Putin party seen winning even greater sway in Russia's parliament

It is also a test of how well the Kremlin can oversee trouble-free elections. It will be the first parliamentary vote since 2011, when allegations of ballot-rigging sparked big protests against Putin in the capital.

United Russia, led by staunch Putin loyalist, Prime Minister Dmitry Medvedev, has 238 of 450 Duma seats, dominates the more than 80 regional parliaments, and is routinely depicted in a favorable light by state TV, where most Russians get their news.

The party is also able to draw on the support of the other three parties in the federal Duma, and benefits from its association with Putin, who after 17 years in power as either president or prime minister, enjoys a personal approval rating of about 80 percent.

By contrast, liberal opposition politicians, who have just one sympathetic member in the Duma, complain they are starved of air time, vilified by state media, and their campaigns systematically disrupted by pro-Kremlin provocateurs. Pro-Kremlin politicians deny that charge.

September 18's vote will be closely watched to see how many of Russia's roughly 110 million registered voters across 11 time zones turn up to cast their ballots, with some opinion polls showing apathy levels are high.

Putin has said it is too early to say if he will go for what would be a fourth presidential term in 2018. If he did and won, he would be in power until 2024.

Observers from the Organization for Security and Co-operation in Europe are being allowed to monitor the vote, a new head of the central election commission has been appointed, and regional and Kremlin officials have been fired in the run-up.