MEXICO CITY, Dec 30 (Reuters) - Mexican oil and gas monopoly Pemex said on Sunday a drunken group caused a ruckus outside an installation overnight, but denied it was another sabotage attempt to cut gas or oil supplies.

On-line newspaper El Universal said it was a failed third guerrilla attack by a shadowy leftist rebel group on Pemex installations. Rebels blew up pipelines in September and bombed other parts of Mexico's oil infrastructure in July.

But Pemex said it was not a sabotage attack and the installations in central Guanajuato state had not been damaged in the incident.

"It was a group of people who were causing a scandal. They were drunk," a Pemex spokeswoman told Reuters.

The Popular Revolutionary Army, or EPR, exploded gas and oil pipelines in early September, cutting off a quarter of the country's natural gas flows for days in its biggest attack on economic targets since emerging in the mid-1990s.

The EPR, believed to number 1,000 members and with political grudges against the government, launched a campaign of economic sabotage in July with bomb attacks on other energy installations.

The September blasts caused hundreds of millions of dollars of damage to Pemex and thousands of businesses.

The group, which also calls for land reform and a socialist state, also wants the government to give up two rebels it says were taken by security forces. The government denies taking the two guerrillas. (Reporting by Adriana Barrera and Chris Aspin, editing by Maureen Bavdek)

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