Fed up with morning rush hour delays, angry commuters in Argentina have set a train on fire and pelted it with rocks.

The government has blamed the incident on leftist activisits.

Black smoke and flames engulfed the train at the station of Merlo, in the western suburbs of the capital, Buenos Aires. At nearby Castelar, passengers hurled stones at the ticket office and blocked the rails.

Many commuters said the delays, caused by a broken-down train, had cost them a day's work.

Argentina's Justice Minister Anibal Fernandez said the faulty train's brakes had been sabotaged and leftist political activists took rocks and flares from their backpacks to incite violence and set the train aflame.

Fernandez told a news conference: "This was planned, it was premeditated."

He said the fire had caused nearly $7 million in damage to the newest train running on the heavily traveled line.

Police arrested seven people on theft charges during the incidents, he said.

Fernandez accused militants of the Workers Party — a political arm of organized unemployed workers known as "piqueteras" — of planning the attack.

The Workers Party denies the accusations and blames the government for trying to cover up legitimate protests over public transportation.

Argentina's dilapidated rail services are plagued by delays and commuters' anger has erupted into violence before.

Last year, commuters torched a carriage at a station south of the capital and rioting broke out at a main railway station when passengers clashed with police, causing dozens of injuries and arrests.