Archenemy motorcycle gangs opened fire on each other in a crowded casino southeast of Las Vegas early Saturday, leaving three slain and at least a dozen hurt.

Frightened gamblers dropped to the floor to take cover amidst the shower of bullets.

Denise Massey, 48, was gambling with her fiancé 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 arrested one person and recovered a handgun, said Sgt. Chris Darcy, spokesman for the Las Vegas Metropolitan Police Department, which patrols the Colorado River gambling resort 80 miles from Las Vegas. Knives were also involved in the attacks, police spokesman Lt. Vincent Cannito said.

Cannito also said the police were in the process of interviewing about 200 people. The Hells Angels and the Mongols were the two gangs involved, Cannito said. The groups are long-time rivals.

Police did not yet know which group started the fight and did not yet release the affiliation of the victims or suspects. "We're trying to separate the victims from the witnesses and that's all we're going to say about the investigation," Cannito said.

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.

Tunforss had said early in the day that four were killed, but police later revised the figure.

Authorities took one person into custody and locked down the city, temporarily closing bridges over the Colorado River. The routes were reopened later in the morning.

The shooting occurred during the 20th annual River Run, a weekend gathering of motorcycle clubs that brought up to 80,000 people into Laughlin.

"Every year for approximately the last 20 years, the Laughlin River Run has taken place without a hitch," Cannito said.

Harrah's director of marketing, Laura De La Cruz, said a fight between rival motorcycle gang members erupted shortly after 2 a.m. She would not give any other details.

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," she said. "Everybody fell to the floor. It was crazy."

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

Later in the morning, authorities 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. It was not immediately known if the death was related to the casino shooting, San Bernardino County Sheriff's spokesman Chip Patterson said

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 was treating seven fight victims. Two were in critical condition and undergoing surgery, while the other five were listed in fair condition.

The Associated Press contributed to this report.