Saboteurs thought to be left-wing rebels detonated six bombs along a major pipeline over the weekend, halting petroleum production at Colombia's second-largest oil field in the most serious such episode this year, officials said today.

Authorities speculated that the guerrillas took advantage of the fact that army forces had been temporarily reassigned from the Canon Limon-Convenas pipeline in northeast Colombia to provide security for Sunday's presidential runoff election.

The attacks were blamed on rebels of the National Liberation Army, which has been accused of hundreds of such bombings, as well as numerous kidnappings and killings of oil workers. The blasts caused four ruptures and two large dents along the 430-mile line, which is operated by the state oil company, Ecopetrol.

Occidental de Colombia Inc., a subsidiary of Occidental Petroleum Corp. of Bakersfield, Calif., which runs the Cano Limon oil field in Arauca province near the border with Venezuela, was forced to suspend oil production there Sunday because of limited storage capacity after pipeline operations were halted. Repair work, which officials said could take several days, is a complicated and risky undertaking that involves securing work areas against guerrilla attack and sweeping for land mines. This shutdown came on the heels of one last week, also as a result of guerrilla violence, and has exacerbated the current storage problem.

Colombian rebels have targeted the oil sites for more than a decade, disrupting production of foreign oil companies that have invested heavily here. But since petroleum overtook coffee in 1996 as Colombia's foremost legal export, rebels have stepped up their efforts to undermine the industry. The attacks have also adversely affected regional ecosystems by unleashing huge amounts of oil onto surrounding lands.

"This is causing damage to the environment, it is affecting the . . . economy and compromising employment for workers and contractors," said Robert Stewart, spokesman for Occidental de Colombia.

Last year, there were a record 65 bombings along the pipeline, which snakes from Arauca to the Caribbean coast. This year, there have been 32.