RIO DE JANEIRO, July 30 (Reuters) - Brazil's interim president, Michel Temer, presided over the Saturday opening of the last major infrastructure project to be finished for the Olympics, the much-delayed extension of a metro line completed just six days before the event begins.

The 10 billion-real ($3.1 billion) expansion of the metro from Rio's Ipanema neighborhood to Barra, the area housing the Olympic park and village, is key to the smooth transport of fans and athletes between the games' different competition zones.

The metro can only be used by athletes and delegation members, fans with tickets for events the day they travel, and others with Olympic credentials through the Games. It will open to the public Sept. 19, the day after the close of the Paralympics.

At the ceremony marking the opening of the metro, Temer told reporters he hoped the Games would help unite a bitterly divided Brazil, which is facing political and economic crises, and has suffered repeated stumbles in its run-up to hosting the world's biggest sporting event - including having less than a week to test the extended metro's operations.

Temer is widely expected to officially take over the presidency just days after the Olympics end on Aug. 21. Suspended President Dilma Rousseff is facing an impeachment trial over alleged budget irregularities, and the Senate is expected to vote against her the last week of August.

Temer would then take over until elections in 2018. Polls show he is as disliked as Rousseff, with approval ratings in the single digits, a fact he tacitly acknowledged this week by saying he was prepared to be heavily booed when he attends the opening ceremony of the games on Friday.

The last-minute completion of the 16-kilometer (10-mile) metro extension, which was first planned in 1998 but repeatedly delayed, was not the only challenge the project has faced: Federal investigators are probing its construction for possible corruption.

The consortium that expanded the metro is led by Queiroz Galvao and Odebrecht Infraestrutura - two firms caught up in Brazil's biggest corruption scandal yet uncovered, a sprawling kickback scheme at state-run oil company Petrobras.

Still, the metro is expected to improve Rio's increasingly congested traffic. The expanded metro is forecast to carry an additional 300,000 passengers per day, according to a recent study from the Getulio Vargas Foundation, a top Brazilian think tank and business school.

(Reporting by Rodrigo Viga Gaier; Writing by Brad Brooks; Editing by Mary Milliken) ((Brad.Brooks@thomsonreuters.com; +55-21-9-6749-5042; Reuters Messaging: brad.brooks.thomsonreuters.com@reuters.net))