Russian President Vladimir Putin on Saturday inaugurated a $2 billion Moscow railway ring project that is opening for passengers just before parliamentary elections.

The completion of the project, pushed back from 2015, comes as Russia is struggling to plug holes in the budget amid lower oil export revenues and Western sanctions over Ukraine.

Russia holds elections to the lower house of parliament on September 18, and, despite the economic crisis, the main pro-Kremlin United Russia party is expected to win comfortably, though its margin of victory could be slimmer than in recent years.

Still not fully finished, the 54-kilometre (34 mile) Moscow Central Ring - an aboveground metro railway connecting the suburbs of the capital - opened for passengers on Saturday.

The government has invested around half of the 130 billion rubles ($2.01 billion) cost of the project, aimed at easing pressure on the Moscow metro, one of the world's busiest.

The rest has come from the Moscow city budget and state railway monopoly Russian Railways.