A massive weekend inferno apparently sparked by welding in a mattress factory ravaged part of a leafy commuter town in North Jersey — wreaking havoc as its police department lost power and schools were forced to shut down Monday.

“It was a perfect storm — we were staring into hell last night,’’ Belleville Mayor Michael Melham told The Post on Monday — as the “12- to 14-alarm’’ blaze in the Essex County town’s Valley section was finally contained but continued to smolder more than a day after it ignited.

“It’s an absolute miracle we had no deaths,’’ the mayor said.

The flames erupted around 3 p.m. Sunday in one of a string of closely connected buildings on Joralemon and Cortlandt streets in the town’s industrial section, Melham said.

The fire may have been caused by “some welding happening inside of a mattress factory — which is never a good combination, obviously,’’ the mayor said.

“We suspect” that’s the cause, but it’s still under investigation, he said — adding that arson investigators and detectives will determine if there was potential criminality and whether the buildings were up to code.

Dramatic video reportedly from surveillance in the warehouse and obtained by PIX 11-TV showed a small fire begin in a corner near a stack of mattresses, cardboard boxes and wooden pallets — with a nearby worker not noticing the flames for about 15 seconds.

The worker then rushes over and tries to get the mattresses away from the growing blaze but is only able to barely cart one off as the whole area begins to catch fire. 

Another worker is alerted and tries using the equivalent of a skinny garden hose to spray water on the flames, but the fire quickly grows out of control, and within a little over a minute, the footage is clouded by smoke.

Sprinklers did not appear to kick on in the footage. Melham said at least one of the burned buildings had sprinklers, although he added that doesn’t mean it was necessarily still up to code.

PSE&G, which provides the area’s gas and electricity, cut power to the area around the blaze as a precaution, Melham said. Water was also turned off at some residences.

“One of the unintended consequences of that was our police department about three blocks away was down last night,’’ he said.

The mayor said the town’s contingency plan kicked in to handle 911 calls — moving dispatch to neighboring Nutley — and “there was never any threat or concern to residents.’’

But the flames took out nearly a whole block as the fire jumped from roof to roof, and “flying embers” even ended up burning two homes “multiple blocks away,” Melham said.

The township’s large school district — which has more than 5,000 students — was closed Monday because “they lost some fiber optics” from the heat, he said.

“Their infrastructure technology-wise was temporarily down,” the mayor said. “Plus, we have so many road closures right now. You can’t drive” because there are so many firefighting vehicles and equipment in the area, as well as multiple closures of major thoroughfares.

The district announced Monday evening that all schools — with the exception of School 9 down the block from the fire scene — will be open Tuesday and operating on a normal schedule.

Between 40 and 50 departments responded to the local catastrophe, including some at least an hour away, but that also caused another issue: lower water pressure, as the emergency responders vied for supplies to help put out the blaze.

“We were fighting wind, we were fighting the fire. … When you have 50 different departments trying to tap into [the water supply], you run out of pressure very quickly,’’ Melham said.

The mayor said he issued a state of emergency, which provides more resources for the town from entities such as the county and state.

He said the blaze was under control but still “active” Monday, adding he hoped it would be extinguished by later Monday or Tuesday.

By early evening, it was “still smoldering. We’re about 90% complete,” Melham said.

An air quality alert was issued at one point in nearby North Arlington because of the smoke, with residents there urged to close their windows and remain indoors, according to NJ.com.

Lyndhurst officials also warned their residents of smoke issues.