CHELSEA — Firefighters raced inside a burning apartment building Wednesday morning to rescue a 3-year-old boy from a four-alarm blaze that badly damaged more than 20 apartments and injured two firefighters, officials said.

Chelsea Deputy Fire Chief John Quatieri said firefighters received word that a toddler was trapped in an apartment above the fire when they arrived at the four-story brick building on Eleanor Street around 10 a.m.

“We sent a crew up and we were able to get him down through the interior before the fire spread,’’ Quatieri said.

The child was unharmed and did not need medical attention.

Two firefighters were injured in the blaze. One sustained a back injury and was sent to the hospital, and a second sustained a shoulder injury. He was treated at the scene.

The fire started in an apartment on the third floor and quickly spread, Quatieri said. It took firefighters more than an hour to put out the flames.

“The fire got into the walls, the ceiling, and the chase was on at that point,’’ Quatieri said.

The fire caused extensive damage to the building, Quatieri said. The windows in several upper-floor apartments were shattered, and the exterior of the building showed obvious smoke damage.

The Red Cross assisted the residents of the 24 apartments. Between 50 and 100 people were displaced.

“They’ll be out for a while [because of the smoke and water damage,]’’ Quatieri said. “It doesn’t look like anybody’s going to be allowed to come back in.’’

The state fire marshal’s office and the Chelsea Fire Department are investigating the cause of the blaze.

Quatieri said the sprinkler system in the building did activate, but was not able to control the fire.

“Once a fire gets into a wall or a ceiling it’s beyond the control of a sprinkler system,’’ Quatieri said.

The sprinkler system did give residents time to flee the building, he said.

As light rain fell on Eleanor Street Wednesday, some residents spoke with firefighters, while others stared at the empty building.

“I was scared, that’s it,’’ said Mimi Sanka, who was in her second-floor apartment when the fire started. “I was sleeping with my baby at the time.’’

She awoke to the sound of fire alarms and a friend’s knocks on the door, she said. She wasn’t sure where she would go.

“We’re going to see,’’ she said.

Another woman, who lived in an apartment near the source of the fire, said smoke filled her kitchen.

“I stopped what I was doing and I left,’’ she said.

A woman who lives across the street from the apartment building said the fire was intense.

“The smoke was very, very bad,’’ said the woman, who asked to remain anonymous. “This is the first fire I’ve seen on my street, and I’ve lived here for 30 years.’’

Mina Corpuz contributed reporting. J.D. Capelouto can be reached at jd.capelouto@globe.com. Follow him on Twitter @jdcapelouto.