A large bomb exploded in a blast of smoke and debris yesterday afternoon in a Stateline, Nev., casino as demolition experts, using a robot, attempted to disarm it. The explosion, capping nearly two days of suspense, came just hours after an unsuccessful attempt to pay extortionists $3 million, the FBI said.

Five hours after the explosion in Harvey's Resort Hotel-Casino, the first gambling house in the Lake Tahoe area, a fire broke out on the mezzanine level. The flames were extinquished quickly, and no injuries were reported in either the explosion or the fire. The cause of the blaze was not immediately determined.

Damage to the casino from the explosion alone was estimated unofficially in the millions of dollars.

Witnesses in nearby casinos said they felt the ground shake, smelled a "gunpowdery" order and saw smoke rolling down the neon-studded main street of the gambling spa.

Douglas Country Sheriff John Maple told an Associated Press reporter that bomb specialists were trying to "render [the bomb] safe" by remote control when the explosion occurred at 6:43 p.m. (EDT).

At 6:30 p.m. (EDT), reporters and others in the vicinity heard Sheriff Maple on the police radio, warning that the disarming attempt was about to begin.

"This," he said, "is a 15-minute notification."

Reporters and others took cover behind rocks, walls and other objects, and waited. Ten minutes later, Maple came on the radio again:

"In approximately five minutes, we will have an explosion in the Stateline area, coming from Harvey's."

And then, minutes later:

"When the siren quits, and until after the explosion is quiet, keep down and don't look at the hotel. Watch out for flying glass. . . ."

Silence. Then the siren. And the blast.

"It was the only thing we could do," Maple said, adding that the casino's owner, Harvey Gross, agreed with the decision.

Maple, in a brief news conference after the bomb went off, said the device was "considerably smaller" than had been feared, but still "very, very violent," the Los Angeles Times reported.

Maj. Michael Christy, an Army explosives expert, said that the explosion occurred while efforts were under way to separate the ignition system from the explosives. He said ordnance experts had hoped to move the explosives out of the building when that was completed.

But a three-page typewritten extortion note found with the bomb Tuesday had warned that any attempt to move it would result in detonation -- and that, Christy said, appeared to be just what happened.

"It was, he said, "a very big and very sophisticated device."

Maple refused to discuss whether an attempt had been made to meet the extortionists' demands.

But other sources told the Los Angeles Times that Gross had followed instructions in the letter and had placed the $3 million demanded on a helicopter, ready for delivery, Tuesday night.

This attempt, however, came to nothing when the helicopter pilot failed to see the prearranged signs at the delivery point as described in the letter, and returned with the money, the sources said.

Reporters in the area described the blast as "like a sonic boom," preceded by a shuddering of the casino's lower walls.

"The whole front went out," one security guard told reporters.

Windows were shattered and room balconies wrecked in the vicinity of the bomb, the Los Angeles Times reported. The device was located in the complex's second-floor executive suite, and the building concrete wall was pierced there.

Harvey's had been evacuated for nearly 24 hours before the explosion, and four other casino-hotels in the area had been partly evacuated.

Windows and other vulnerable items facing the endangered hotel had been boarded up or removed, but a number of guests in hotels near Harvey's refused to leave.

At the Sahara-Tahoe Hotel, a crowd of frustrated gamblers had been hearded into a lobby on the far side from neighboring Harvey's. Some screamed when they were startled by the explosion, according to United Press International, but then they cheered.

Those on the scene reported seeing within minutes T-shirts that read "I Had A Blast At Harvey's," or "Get Bombed At Harvey's." An NBC observer called it a "carnival atmosphere."

The mysterious bomb, said to contain 1,100 pounds of TNT, plus switches and "tiltmeters," had been smuggled into Harvey's, a 12-story, 250-room casino valued at $20 million, about 5:30 a.m. Tuesday by two men disguised as computer technicians in overall, officials said.

Found with the bomb was a type-written demand for $3 million and a helicopter in exchange for instructions on how to disarm the device.

A small army of explosive experts gathered in Stateline, about 40 miles south of Reno, and spent nearly two days X-raying the device, tying monitoring wires to it and trying to decide whether to try to disarm it, detonate it or leave it alone.

Nevada Gov. Robert List had issued a midnight appeal to the extortionists for clarification of their directions and indicated that the hotel management was "prepared to comply."

The episode in this resort city on the shores of picturesque Lake Tahoe at the California-Nevada border took on the dimensions of a high-stakes crime thriller after about 3,000 gamblers and guests of Harvey's were hustled away from the gambling tables or out of their beds Tuesday morning.

The FBI, the Army and the Department of Energy all contributed bomb specialists. Authorities sealed off a 3/4-mile area around the casino, causing massive traffic jams.

Most -- but not all -- of the most diehard gamblers had been persuaded to leave several other casinos near Harvey's as the day wore on.

X-rays had determined postively -- "based on shapes and densities which were fed into a sophisticated computer," as sheriff's officers put it -- that the device was a real bomb. Still, speculation had continued that the whole thing was a hoax.

Authorities played it close to the vest all along, at first trying to persuade reporters not to mention that there was a bomb. Later, they offered a minimum of information about what was going on. At midday, Sheriff Maple said vaguely that, "We've held up our end of the bargain but they [the extortionists] haven't."

The unknown extortionists had rolled the explosives into the casino under a blanket that bore the letters "IBM," and the TNT was housed in an exotic array of 28 switches and tiltmeters (devices that measure the levelness of an object), according to several sources.

Despite free food and drinks supplied by casinos in the area, some tourists complained that they had been displaced from their rooms and that their vacations were ruined.

But the more prevalent mood was captured by Tracy Heaton, 25, of Los Angeles, who told a reporter she wasn't even fazed when a hotel worker burst into her room early Wednesday to warn her a bomb was about to go off.

"I'm originally from New York, and this happens all the time in New York."