A chain-reaction explosion ripped through a 90-silo grain elevator at the Continental Grain Co. plant in suburban Westwego today, leaving at least six persons dead and more than 20 missing.

Most of them were feared crushed in the one-story control building and lunchroom, smashed flat when a weighing station perched above the the tops of the silos shattered and fell 250 feet on top of it.

"We don't have much hope for some of those men," said Harlan Ryan, division manager of the federal grain inspection service.

More than 50 men were working at the elevator when the blast happened, but many apparently escaped uninjured. Eleven were taken to hospitals, most suffering from severe burns.

Rescue teams were hampered by heavy smoke and dust as they looked for bodies in the concrete rubble at the bank of the Mississippi River.

Cause of the blast was not known, but dust-filled air inside the cylindrical grain elevators is volatile.

"I was standing on the ship a couple of hundred yards away from the main head house," said Michael Gavron, 24, of Wilmington, Del. "I heard an explosion and turned around. The only thing I saw was big flames jumping up from the top of the whole elevator.

The huge elevator had a capacity of 6 million bushels and the explosion apparently occurred while a Norweigian ship was being loaded, authorities said.

In one of the more dramatic rescues, a Coast Guard helicopter dipped down into the smoke and flames to lower a basket to a man trapped on top of the 130-foot-tail elevators.

Two inspectors were blown out of the weighing office to the ground. They were in critical condition.

The blast roared through two-thirds of the clustered silos, which sit 100 yards from the Mississippi. The tops of the cylinders were blown out, and the heavy concrete walls were shattered, leaving the giant honeycomb of tubes leaning askew.

It was raining concrete," said Allus Foret, 52, a workman who was outside the elevator and was knocked down by the blast.

Jefferson Parish Sheriff Alwynn Cronvich said it might take all night to recover the bodies from the twisted ruble of the plant, located nine miles upriver from downtown New Orleans.

The explosion rocked the small town of Westwego and showered the area with grain dust and heavy debris. Across the Mississippi at the Audubon Park Zoo, animals cried and roared hysterically.

Fireboats joined the Westwego volunteer fire department and equipment from nearby areas at the plant. Ambulances ringed the facility and Coast Guard helicopters hovered over the elevators.

FBI agents were called to the scene because federal employees were believed to be among the victims. The federal government earlier this year took control of all grain insepctions along the lower Mississippi because of a scandal involving shortweighting of foreign grain shipments.

The explosion, which occurred at mid-morning, shook walls and rattled windows as far as 10 miles away. Some witnesses said the blast "looked like an H-bomb" and sent thick, black smoke 2,000 feet in the air.

Earlier today, meanwhile, an explosion tore through steel-reinforced concrete walls and ripped apart sheet metal partitions at a Tupelo, Miss., mill, sending at least 18 people to hospitals, some of them critically burned, authorities said.

The explosion occurred in a dog food manufacturing section of the Sunshine Mills, Inc., plant. Police Chief Ed Crider said it may have been caused by "a combination of grain dust, maybe set off by an electric motor or electric welder or something of that nature."

The most severely injured workers were flown to burn centers in Jackson, Greenville and Memphis.

John Perkins, who was working on a loading dock beside the building at the time of the blast, said it would be impossible to describe the explosion.