LAS VEGAS (AP) _ A smoky fire touched off by a welder's torch raced through a new casino under construction Thursday at the Riviera Hotel and Casino, forcing the evacuation of at least 3,000 gamblers and guests.

Only two minor injuries were reported in the 10:20 a.m. blaze, which burned curtains and broke windows in up to 75 rooms in the hotel before being stopped by automatic sprinklers. Damage was estimated at about $3.5 million.

Gamblers and guests were evacuated without incident when smoke began filling the inside of the resort. The hotel patrons joined several thousand spectators on the Strip watching the fire, which was put out in about two hours.

''The first thing we did was secure all of the money (in the casino) and evacuate all the people,'' hotel President Mark Sterbens said. ''To my knowledge it's the first time we've ever shut down gambling at the Riviera.''

Arthur Waltzman, Riviera chairman, said he went floor to floor with security officers, knocking on doors to alert hotel guests as corridors filled with smoke.

''Everything worked the way it was supposed to work,'' Waltzman said of the hotel's sprinkler and fire alarm systems.

The casino, which suffered water damage, reopened about 9 p.m. Thursday, and a performance by Paul Anka was delayed until 10 p.m. Hotel officials invited all guests to a free prime rib buffet and offered them free tickets to Anka's performance.

The fire blackened the exterior of a section of the original hotel's nine- story structure built in 1953. The fire reached inside 50 to 75 rooms in the building, fire officials said, before being stopped by automatic sprinklers.

The fire began in wooden forms being used to build a new 125,000 square- foot two-story casino touted as the world's largest. The blaze also spread to the eaves of an existing casino area and burned part of an outdoor sign on the casino. Embers fanned by 25 mph winds also crossed the street and briefly set the roof of a motel on fire.

Two other high-rises at the Riviera, where most of the hotel's rooms are located, were not affected by the fire but were evacuated anyway.

Clark County Fire Department spokesman Bob Leinbach said the blaze was touched off by welders who were building a swimming pool on top of the new casino. Leinbach said firefighters later found two empty fire extinguishers nearby.

Sterbens said the hotel's 2,200 rooms were sold out. The hotel is hosting a convention of the American Association of Spinal Cord Injury Nurses and Psychologists.

Nevada enacted some of the most stringent fire safety codes in the nation after a fire at the former MGM Grand Hotel in November of 1980 killed 87 people and a blaze at the Las Vegas Hilton three months later killed eight people.