A huge explosion rocked Venezuela's biggest oil refinery Saturday morning, killing at least 39 people and injuring more than 80 others in the deadliest disaster in memory for the country's key oil industry.

Balls of fire rose over the Amuay refinery, one of the largest in the world, in video posted on the Internet by people who were nearby at the time.

At least 86 people were injured, nine of them seriously, Health Minister Eugenia Sader said at a hospital where the wounded were taken. She said 77 people suffered light injuries and were released from the hospital.

Officials said those killed included a 10-year-old boy, but that most of the victims were National Guard troops stationed at the refinery.

"People are very scared," said Falcon State Gov. Stella Lugo, according to the state-run AVN news agency. "They evacuated the areas that had to be evacuated, but the situation, as the technicians are telling within the refinery, is controlled."

President Hugo Chavez decreed three days of mourning and gave his condolences to families of the victims.

"This affects all of us," Chavez said by phone on state television. "It's very sad, very painful."

Chavez said he ordered a "deep investigation" to determine what caused the explosion.

Rafael Ramírez, president of Pdvsa, the state-owned oil company that runs the refinery complex, said the blast, which occurred at 1:15 a.m., was caused by a gas leak.

Workers at the Amuay refinery detected the buildup of a cloud of gas and began to respond, Mr. Ramírez said, but within moments the blowout occurred.

“All this happened very fast, and the explosion occurred almost immediately,” Mr. Ramírez said in televised remarks shortly after 9 a.m.

Venezuela is a major oil supplier to the United States. It was not immediately clear how operations at the refinery would be affected by the explosion. Mr. Ramírez said he expected work there to resume in two days.

Mr. Ramírez said that the explosion damaged a group of storage tanks and caused severe damage to a National Guard post at the site.

The Amuay facility is part of the Paraguaná refinery complex, one of the largest in the world. The complex can process about 900,000 barrels of crude oil a day.