Three firefighters were blown from the roof by the explosion, said Dave Imming, a spokesman for Durango Fire & Rescue Authority.

Rescue Authority Deputy Chief Allen Clay, and firefighters Nathan McGrath and John Brennan were admitted to Mercy Regional Medical Center for observation, said David Abercrombie, DFRA spokesman. Four others were treated and released from Mercy as of Friday night. The injuries were not life-threatening.

The fire was reported at 1:39 p.m. after smoke and flames were seen coming from the roof of Seasons Rotisserie & Grill, Abercrombie said.

At approximately 2:30 p.m., the storefronts of Seasons, 764 Main Ave.; Half-Price Tees, 758 Main Ave.; and Le Rendezvous, 750 Main Ave., exploded in a fireball, showering the street with flying glass, bricks and other debris.

"It looked like the whole roof of Le Rendezvous rose up," said Tom Barnes, who witnessed the explosion from a roof on East Second Avenue. "The lead guy at the hose went pinwheeling. He disappeared into the smoke and we couldn't see him."

Cell-phone service in the area was temporarily disrupted, as were traffic lights. Several hundred people were on the 700 block watching the fire at the time of the explosion, but the area was cleared immediately afterward.

Nearby business owners carted out papers and other valuables in case the fire spread.

Main Avenue was closed from Seventh Street north to Ninth Street and from East Second Avenue west to Narrow Gauge Avenue.

All off-duty fire personnel were called in almost immediately after the first report. Durango Fire & Rescue was the initial response agency, and the Upper Pine Fire Protection District, Los Pinos and Fort Lewis Mesa fire departments also responded.

Paige Marchus, a server at Seasons, said a kitchen cook told her that the hood exhaust fan had stopped working, apparently starting the fire in a vent above the kitchen. It spread quickly to the adjacent businesses. All businesses in the 700 block were evacuated.

The fire was out as of Friday evening.

It may be a day or more before investigators release the official cause of the fire, Abercrombie said. He said the gas in the building had not been turned off, and speculated that it could have been the cause of the explosion. Abercrombie left open the possibility that the explosion was caused by a backdraft combustion - where oxygen-starved blazes suddenly reignite - but said several firefighters indicated they did not see typical signs of a backdraft.

Karen Barger, owner of Seasons, watched from the alley east of Main Avenue as her business was destroyed. She expressed relief that her employees and diners were OK. "Everybody got out," she said.

Barger lamented that the restaurant's wine collection was destroyed. "We had a rather large wine inventory with some things I cannot replace," she said.

Eight to 10 employees and about 16 diners were in the restaurant when it was evacuated, including Sherry Exum of Durango. She said patrons saw a downdraft of smoke at the top of a window outside. No one thought much of it, she said, but then someone reported there was a fire in the building.

"There were a lot of people questioning: 'Do I pay or what do I do?'" she said. "We didn't know the extent of it."

Exum left the restaurant and saw only a small column of smoke. "You would be surprised at what it looked like when it started. It was nothing."

Also in the restaurant at the time was Joe Hawley of Durango. "Someone inside said the roof is on fire, and I said I'm out of here," he said.

In the alley behind Seasons, Hawley saw smoke seeping from power fans that were part of a stove, he said. The fire then spread to the roof, and the T-shirt shop caught fire. "You generally can't do so much to save these old buildings when the roof is engaged," he said while looking on. "This is a huge loss."

All employees of Seasons were given a free dinner and drinks at Guido's Favorite Foods, just down the street.

For some residents, the fire brought back memories of the Central Hotel Fire, which burned 24 apartments above El Rancho Tavern on July 29, 2006.

"It's really sad to see someone else's business be affected by fire," said Chip Lile, owner of El Rancho, which closed for several months after the Central Hotel Fire. "It brings back bad memories."

In 1974, an arson fire in the 800 block of Main destroyed nine businesses and several apartments. Firefighter Nick Parks III and Durango Police Cpl. Gale Emerson were killed when a brick wall exploded.

Jim Guarasci of Durango saw Friday's fire in its early stages. Suddenly, there was a loud explosion, and three firefighters who were on the roof of Seasons fell to the ground. One jumped right up; the other two were carried away on stretchers.

"I was paying attention to the smoke and all of a sudden, whoosh, and three firemen came off the roof," Guarasci said. "It happened so fast."

A firefighter asked to use Guarasci's cell phone, and he called someone to say one of the three firefighters was OK, but was going to be taken to Mercy.

Onlookers were kept back from the scene of the fire. Many of them had camera phones aimed at the blaze. One viewer, Tom Abbott, a real-estate agent in Bar Harbor, Maine, watched online through Brainstorm Internet's "Eye on Durango" Web cam in Durango. "The first thing I saw was smoke coming out of the roof of one building," Abbott said from Maine. "There were no firetrucks there. But after they arrived, it seemed that the fire began to spread." Brainstorm's Web cam crashed almost immediately because of heavy online traffic, said Brainstorm owner Phil Bryson. "We had almost a couple thousand hits almost immediately, especially when the television stations started carrying the feed," he said. The site was back up at 4:44 p.m.

Heavy online traffic also clogged The Durango Herald's Web site, which temporarily shut down about 5:40 p.m.

Police expect the 700 block of Main Avenue to remain closed Saturday.

Barry Mason, owner of the building that houses Half-Price Tees, looked on as his building was gutted. He expressed concern for the owners of the T-shirt shop.

"Buildings can be rebuilt, lives are priceless," he said. "It's a mere inconvenience for me. It's a loss of their livelihood."

Rob Blythe of Durango called the fire "heartbreaking."

"It's terribly sad," he said. "It's like the second huge fire in a couple of years.

"People are just stunned. You can see it on people's faces."

Bonnie Doolittle, who works at a nearby real-estate office, took away wine-stained corks as a memento.

"When they rebuild, they can have these," she said.

Staff writers Shane Benjamin, Karen Boush, Dale Rodebaugh, Tom Sluis and Nancy Richmond contributed to this report.