Multiple vehicles including Mercedes went up in flames in the parking lot of the Kings Plaza Shopping Center in Brooklyn on Monday morning, injuring 21 people and sending plumes of thick black smoke billowing into the sky, officials said.

A 911 caller reported a car on fire on the second floor in the four-level parking garage at the mall in Marine Park around 8:30 a.m., FDNY Commissioner Daniel Nigro said at the scene.

When firefighters responded, they found several vehicles on fire, according to officials. The blaze then spread to the third floor, where more cars were burned up.

Nigro said as many as 120 cars were being stored in the garage by local car dealerships.

“We’re not sure how many of them burned, but many of these cars were involved in this fire, creating a huge amount of black smoke,” Nigro said.

Conditions from the fast-moving blaze worsened and by 9:50 a.m., the inferno was elevated to a six-alarm blaze. The fire was eventually upgraded to a seven-alarm.

More than 250 smoke-eaters battled the blaze, which was primarily extinguished shortly after 11 a.m.

“The black, toxic smoke, the heat, the limited area firefighters had to work in all made it very difficult to quickly extinguish the fire,” Nigro said.

The fire caused non-life-threatening injuries to three civilians and 18 firefighters, officials said. All victims were in stable condition.

“Our members have various levels of smoke inhalation and heat exhaustion, working under those conditions with this amount of smoke and this amount of heat wearing what they wear, one can only imagine what they went through,” said Nigro.

The cause of the fire remains under investigation.

When asked about potential arson, Nigro said: “It’s too early to even look at that yet.”

“It’s common for auto dealers to store their vehicles somewhere off their sales premises. They’ve been storing vehicles here for quite some time,” Nigro said.

A mall security staffer said car dealerships for Dodge, Mercedes, Toyota and Nissan use the garage.

“Most of our cars are in there. I hope we have insurance,” said an employee at one of the dealerships.

Witnesses reported hearing explosions at the time of the blaze.

“I heard, ‘Boom! Boom! Boom!’ ” said area resident Mike Wright. “The next thing I know, fire trucks are coming out. I look up, there is this huge plume of smoke all over the neighborhood over here.”

Then “I heard like 30, 40, it sounded like, cars blowing up.

“There was heavy smoke coming out. Heavy, heavy thick smoke. It sounded like car explosions.”

Nigro noted that any time there is a car fire, “You’re likely to have tires explode.”

“Much of what they heard was tires on these multiple vehicles exploding many times,” the fire commissioner said. “It’s very dangerous for our members and anybody nearby.”

Members of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives also responded to the fire.

Kings Plaza, which was closed to the public at the time but had workers inside, has more than 120 stores including Macy’s, JCPenney and Best Buy. There are 4,000 parking spaces in the indoor garage.

Hali Waldropt, 32, who works at the Adidas store at the mall, said he got caught up in the smoke engulfing the area.

“It was just a cloud of smoke all the way to the corner,” Waldropt said. “People couldn’t breathe. I was standing right in front of JCPenney, and then they started evacuating, and then we all started running back.”

Earlier this month, an anonymous 911 caller made a bomb threat to the Old Navy store at the mall, police said.

The NYPD’s Emergency Services Unit responded to the scene just after 5 p.m. Sept. 5 and determined the threat to be unfounded, cops said.

No arrests have been made in that incident.

Additional reporting by Ben Feuerherd