Laughlin, Nev. ? Rival motorcycle gangs armed with guns and knives clashed early Saturday on a crowded casino floor, leaving three dead and at least 12 wounded as terrified gamblers ducked for cover.

The shooting, the worst ever inside a Nevada casino, occurred during a weekend gathering of motorcycle groups, including the Mongols and Hells Angels.

Bikers wearing Hells Angels colors gather in a parking lot at Gretchen's Inn on U.S. Highway 95 in Bullhead City, Ariz., after other Hells Angels members were involved in a shootout at Harrah's casino in Laughlin, Nev. Rival motorcycle gangs armed with guns and knives clashed at the casino early Saturday, leaving three dead and at least 12 wounded.

Denise Massey, 48, was gambling with her fiance on the first floor of Harrah’s when she noticed 20 to 30 bikers suddenly converge.

“Next thing you know you just hear ‘Bam, bam, bam,”‘ she said. “All of a sudden they’re running and just shooting at each other.”

Police said the violence erupted when a Mongol member walked into a group of Hells Angels. Sixty to 70 people, armed with guns and knives, were involved in the violence inside the entrance to Harrah’s casino, authorities said.

All three of the dead were bikers, police said. One person was arrested and police had recovered several guns. About 200 people were being interviewed, said Lt. Vince Cannito, of the Las Vegas Metropolitan Police Department, which patrols the Colorado River gambling resort 80 miles southeast of Las Vegas.

Authorities also warned of the danger for further violence.

“The rumors of retaliation have already hit the street,” said Larry Tunforss, spokesman for the fire department in Bullhead City, Ariz., which is across the river from Laughlin. “Our police department is beefing up intensely. There’s a definite concern of further incidents.”

SWAT teams and law enforcement officers from across Arizona and Nevada were called in.

The shooting occurred during the 20th annual River Run, a motorcycle gathering which attracted up to 80,000 people in Laughlin. The resort town of about 8,000 residents has nine casinos and attracts about 5 million visitors a year.

Authorities locked down the city after the violence erupted shortly after 2 a.m., temporarily closing bridges over the Colorado River. The routes were reopened later in the morning.

Nevada state archivist Guy Rocha said it was the worst shooting ever inside a Nevada casino.

“In every case that’s been described to me there was no more than one death. This is a threshold,” he said.

Aniko Kegyulics was sitting at a casino bar with her boyfriend when she heard yelling and gunfire.

“There were so many gunshots everything turned gray. Everybody fell to the floor. It was crazy,” she said.

She passed a body covered with a bloody sheet when police allowed people inside to leave at about 8 a.m., she said.

Chris Welton, 42, of Carson City, had been playing poker.

“Shots just started going off,” he said. “There were dead bodies. Pretty nasty. There was a guy about 10 feet from me. He was dead. His buddy was trying to revive him.”

John Davidson, 39, ducked beneath a blackjack table.

“There were shell casings landing right by us,” he said. “I had to pull the dealer down. She was screaming.”

University Medical Center in Las Vegas was treating five patients, three of whom were in serious condition with gunshot and stab wounds, nursing supervisor Louise Burns said. Of the other two, one was in critical condition with a skull fracture and the other was in fair condition with gunshot and stab wounds.

Western Arizona Regional Medical Center in Bullhead City treated seven fight victims. Four were in fair condition and three had been released.

Authorities also were investigating the death of a biker whose body was found along Interstate 40, a main route between Laughlin and Southern California.

The man’s body was found near Ludlow, a small California desert town about 60 miles from the Arizona border. Preliminary reports indicate the man, from San Diego, had been shot while riding his bike and was a member of a motorcycle club, San Bernardino County Sheriff’s spokesman Chip Patterson said.

It was not immediately known if the death was related to the casino shooting.

Traffic in both directions was being diverted off the interstate as sheriff’s deputies investigated, the California Highway Patrol reported. The highway was later reopened.