Update 12:15 a.m.
Firefighters doused the flames that struck a commercial block at 313 Main Street after more than two hours Tuesday, but the cause of the fire has not been determined, Watertown Fire Chief Mario Orangio said.
"Its tough to tell (how it started) because the roof collapsed," said Orangio, who celebrated his ninth anniversary as chief on Tuesday.
See video of firefighters battling the blaze, click here.
Investigators from the State Fire Marshal's Office will look at the building to help determine the cause, Orangio said.
One firefighter went to hospital after suffering a shoulder injury, Orangio said.
"There was an explosion that knocked the hats off a couple firefighters," Orangio said. "They were heading in (when the explosion occurred) and one guy got knocked down."
The building was gutted by the flames. The fire got into the ceiling, and proved stubborn, Orangio said. The building housed Lony Salon, Guy's Hair Shop and Pan World Insurance. Another business, Inner Strength Studios, is attached to the fire-struck building, but did not appear to suffer flame damage.
A tow truck driver spotted the blaze and called 911, so the Watertown Fire Department was able to get to the scene quickly, Orangio said.
The last fire of this magnitude in Watertown was the blaze that struck Lily's Bakery on Galen Street in 2005, Orangio said.
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Update 11:02 p.m.
The Belmont Fire Department has arrived at the scene of the fire at 313 Main St. in Watertown, near the corner of Waverley. The fire appeared close to being knocked down, but one firefighter says when the stream of water stops, flames flare up again.
Update 10:55 p.m.
Fire trucks are still spraying, but there is less smoke. Tweets say odor from fire can be smelled on Moody Street in Waltham.
Update 10:51 p.m.
Watertown Patch Editor Charlie Breitrose says the fire at 313 Main St. looks close to being knocked down. There is a huge puddle of water in the street from all the hoses being put work dousing the flames.
Update 10:43 p.m.
Six streams of water now pouring onto the building at 313 Main St., from three ladder trucks.
Update 10:40 p.m.
Watertown Patch Editor Charlie Breitrose says he now sees five streams of water pouring into the building to fight the flames.
Update 10:34 p.m.
Smoke and flame continue to billow from the building near the corner of Waverley Avenue and Main Street in Watertown. The third ladder truck that arrived is from Boston, joining Watertown's two ladder trucks.
Update 10:28 p.m.
A third ladder truck has showed up, from outside Watertown.
Update 10:16 p.m.
A Boston fire engine has just arrived, as a Watertown ladder truck sprays water from up above. Flames are illuminating the smoke as it billows from the roof.
Original story 10:11 p.m.
Firefighters just smashed in the glass doors of a yoga studio in downtown Watertown, in an effort to reach the flames on Main Street.
Smoke is pouring out of the roof of the commercial building, a one-story brick building with a flat roof.
Guys Hair Shop, at 313 Main St. and Inner Strength Studios, the yoga studio at 309 Main St., are two businesses in the building.
Five Watertown fire vehicles are on-scene, along with two ambulances, and a Newton fire truck just arrived.
No one appears to have been in the building when the fire started.
Watertown Patch editor Charlie Breitrose is on-the-scene. We will bring you more details as we get them.