(Recasts, adds detail on domestic supply)

By Catherine Bremer

MEXICO CITY, July 10 (Reuters) - Mexico tightened security at strategic oil installations on Tuesday after a shadowy leftist rebel group claimed responsibility for a rash of fuel pipeline explosions.

The government described the attacks on a natural gas pipeline in the early hours of Tuesday and on three other pipelines on Thursday as an assault against democracy.

The four blasts shut down pipelines supplying natural gas, liquefied petroleum gas, crude oil and gasoline to the domestic market, but none affected oil exports and there were no injuries, according to state oil monopoly Pemex.

The Popular Revolutionary Army, or EPR, claimed responsibility on Tuesday and said it had begun a campaign against the conservative government of President Felipe Calderon, whose 2006 election win was contested by leftists.

"The order has been given to start a national campaign against the interests of the oligarchy and of this illegitimate government," the EPR said in a statement, adding that it would not give up until the government releases three activists arrested in politically turbulent Oaxaca state in May.

It said it had planted eight explosives in Pemex pipelines timed to go off in the early hours of July 5 and July 10.

The group emerged in 1996 and has been active in the poor southern states of Guerrero, Michoacan, Oaxaca and Chiapas.

Mexico is the world's No. 9 exporter of crude oil and a key supplier to the United States, which values its political stability.

SECURITY MEASURES

The Marxist-inspired EPR has been less active in recent years than in the late 1990s, when it attacked towns in Oaxaca and Guerrero, but Calderon's office said on Tuesday it was taking the threat seriously.

"The Mexican government categorically condemns the attacks on Pemex installations. This criminal conduct tries to weaken democratic institutions," the Interior Ministry said.

"The federal government is taking all the necessary measures to increase security around the country's strategic installations."

Firefighters brought under control Tuesday's blaze at a 36-inch (91.5-cm) natural gas pipeline running between Mexico City and the western city of Guadalajara, and nearby residents were evacuated to safety.

The explosion slowed gas supplies to Guadalajara and other cities in the region and a Honda Motors Co. <7267.T> automobile plant was forced to halt its assembly line.

Pemex said two other pipelines that were shut down by Tuesday's blast served the domestic market with liquefied petroleum gas and sent crude oil to the Salamanca oil refinery, also for the Mexican market. Pemex is a net importer of natural gas and gasoline.

"There is no problem with the (crude oil) pipelines running to ports," a spokeswoman said.

State environmental protection agency, Profepa, said fuel was being burned off to keep contamination to a minimum.

The EPR came out against former President Vicente Fox's conservative government when it came to power in 2000 and analysts say widening economic disparity helps keep the group alive. In 2001, a splinter group known as FARP set off small homemade bombs outside three banks in Mexico City.

Separately, Pemex said it shut off some onshore wells in the southeastern state of Tabasco on Monday because of the risk of nearby grassland fires spreading. The closures trimmed natural gas output by a slight 10 million cubic feet (280,000 cubic metres) per day, Pemex said. (Additional reporting by Miguel Angel Gutierrez, Jason Lange and Noel Randewich)

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