SOMERVILLE — A seven-alarm blaze ripped through a cluster of residential buildings on Calvin Street this morning. Authorities said they believed all the residents managed to escape safely, but two firefighters were injured.

The fire, which spread to six buildings, was discovered around 6 a.m. The fire department called in mutual aid from neighboring departments, including those in Boston, Everett, Chelsea, Brookline, Medford, and Belmont.

Four of the buildings were in flames “before we even got off the truck,’’ Somerville Fire Chief Kevin Kelleher said at the scene.

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An Everett firefighter suffered respiratory problems and a Brookline firefighter injured his knee, Kelleher said.

The fire started somewhere on the back side of the homes. It was not clear whether it started inside or outside, Kelleher said.

It took about two hours to knock down the fire; firefighters spent about two more hours combing the buildings for hot spots, Kelleher said.

When firefighters arrived, they found residents swarming the streets in the area, which is near the Cambridge line. “It was a chaotic scene, there were multiple buildings on fire,” he said. “Our first priority was protecting the residents.”

Two of the buildings were severely damaged, three had moderate damage, and one suffered minor damage, Kelleher said. Inspectors were on scene this morning to determine whether the two buildings that suffered the brunt of the fire would have to be demolished.

Anna Jobe, who lives at 19 Calvin St., said she was awakened by the sound of smoke alarms.

“We just woke up to alarms and it seemed like the back porch was on fire on the second and third floor,’’ said the 27-year-old Jobe. “It went up really quickly. It was ridiculous how fast it went up and how quickly it spread.”

Jobe said she is not sure how much of her possessions survived the flames, the smoke, and the water.

“I think we will probably have stuff left, I just think it will be super-damaged,’’ she said.

But then she paused and became overwhelmed by the stress of her morning. “I can’t even think right now,’’ she said.

Neighbor Bobby Herrera said the sirens of arriving fire trucks did not prompt him to get out of bed, but when he heard loud knocking, he became concerned.

“I was asleep, and I heard a bunch of sirens, but I stayed asleep,’’ said Herrera, a graduate student at Harvard University. “But then I heard knocking and a lot of commotion, so I figured something was wrong.’’

He added, “I looked outside my window at those two buildings — and they were blazing with fire.’’

The massive response by the firefighters left area streets — including Calvin, Caldwell and a major artery, Washington — choked with fire engines, ambulances, and other emergency vehicles. Long ribbons of firehoses crisscrossed the streets.

Kelleher said there had been other fires in the neighborhood recently, but he said the cause of this morning’s fire remains under investigation.

“There are several fires from the last few weeks that are currently under investigation. We’ll look at all of them again,” Kelleher said. “But coincidences happen, and we aren’t ready to label anything yet.”

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