A two-alarm brush fire tore across the banks of the Muddy River Sunday afternoon in the Fenway and sent thick smoke billowing into the sky, drawing onlookers who watched as dozens of firefighters battled the blaze.
Steve MacDonald, a Boston Fire Department spokesman, said the fire started about 3 p.m. and raged for about 30 minutes before firefighters could put it out.
“This thing engulfed the whole [area] in minutes,’’ he said at the scene.
MacDonald said the blaze started behind the fire department’s 911 call center and spread all the way to the Boylston Street Bridge, prompting at least nine trucks to respond. No one was injured.
Though brush fires are often caused by careless cigarette disposal, it is not clear what started the fire on Saturday, and it is unlikely officials will be able to determine a cause, MacDonald said. The wind helped the blaze spread quickly to both sides of the river, he said.
The fire left charred reeds, branches, and tree trunks along its path.
David Smith, 48, of Jamaica Plain, was standing by the river after the fire was put out. He said he had seen ash in the air earlier in the afternoon when he exited the Sheraton Boston Hotel near the Prudential Center.
“The embers, they weren’t lit, but these big, black pieces were floating’’ toward the hotel, he said.
Smith said he then saw fast-moving flames that appeared to be about 40 feet high when he got closer to the river.
“It was like a tornado,’’ he said. “Tornadoes of flames.’’
His friend, Arthur James, 49, of Brookline, said that geese fled when the fire started.
“It was intense,’’ James said.
The men said many onlookers stopped to watch firefighters battle the blaze.
“I’ve never seen so many firefighters,’’ Smith said.
“I’ve never seen so many spectators,’’ James added.
MacDonald said brush fires happen occasionally in the city and that the last one occurred in February along the Neponset River in Dorchester.
Dispatchers inside the fire department building saw Saturday’s blaze on closed-circuit cameras when it started and alerted crews, MacDonald said.
Neighborhood resident Amal Yaalaoui, 35, was with her son at a playground along the river and looked out at the fire’s path after it was extinguished.
“It’s a lot of damage,’’ she said. “A lot of damage.’’
Correction: Because of a reporting error, an earlier version of this story about a fire in the Fens gave an incorrect day for when the fire occurred. The brush fire happened Sunday.