The Boston Marathon bombers were hell-bound for New York City — until eight small town cops, in 12 hair-raising minutes, stood bravely in their way.

In an exclusive interview, the heroes of the 68-officer Watertown Mass. Police Department sat down with The Post to describe, blow-by-harrowing blow, their takedown of Dzhokhar and Tamerlan Tsarnaev.

The two brothers had already killed three and injured 264 Marathon-goers, and had gone on to fatally shoot an MIT cop, when the crackle of police radios at 12:41 a.m. on April 19 heralded their presence in the four-square-mile town just northwest of Boston.

The brothers’ plan? Blow through Watertown, and hit Times Square, authorities since learned — with the younger brother, Dzhokhar, driving a Honda Civic and Tamerlan driving a hijacked Mercedes holding two firearms, five bombs and a terrified hostage.

The brothers didn’t count on two veteran sergeants and six younger officers — one with only four months experience — engaging the desperate brothers in a pavement-shaking, bomb-and-bullet firefight on leafy, picket-fence-lined Laurel Street.

Eight, men, 12 minutes. Some 250 bullets would be exchanged in the firefight, leaving one officer to nearly bleed out at the scene. Three of the department’s police cars would be destroyed by the last of the brothers’ arsenal: two pipe bombs and another rigged pressure cooker like the one that had ripped through the Marathon four days earlier. Two other pipe bombs were flung, but did not explode.

“You think about 9-11 all the time,” one of our Watertown heroes, Sgt. John MacLellan, told The Post.

“New York’s had their share. It’s nice to give them a break for once.”

Watertown’s police dispatcher alerted all units that Cambridge police had traced the GPS of a carjacked, 2013 black Mercedes SUV to their jurisdiction. The vehicle was moving southbound on Dexter Avenue.

“I was in my patrol car, just doing routine patrol,” said Officer Joseph Reynolds, a seven-year veteran of Watertown Police and the first cop out of literally more than 1,000 pursuing officers from a dozen agencies to lay eyes on the two brothers.

But Reynolds and his fellow officers had no idea who they were about to confront.

“I told Officer Reynolds please don’t pull the car over until you have backup, due to the fact that it was an armed carjacking,” Sgt. MacLellan remembered. “The next transmission is Officer Reynolds saying, ‘I have the car. I’m behind the Mercedes.”

“The Civic was in front and the Mercedes was in back,” Officer Reynolds remembered. “As I drove by, I made eye contact with Tamerlan…We both kind of looked at each other. I radioed dispatch that I had the vehicle…All of a sudden they stopped their [cars]. I stopped. I was probably two or three car lengths behind.

Tamerlan got out and started walking toward me. He lifted up his arms and started firing at me. I could hear [the bullets] going off my cruiser.”

Officer Reynolds reversed his cruiser some 30 yards, threw it into park, opened the driver’s side door for cover, and started returning fire. Sgt. MacLellan had been approaching the scene in his own cruiser.

“I got on the radio and said it three times: ‘Shots fired, shots fired, shots fired,’” MacLellan remembered. “As I put [my cruiser] in park, to figure out what we’re gonna do, I saw that [Tamerlan] went from Reynolds’ car to my car and shot, boom, right through the windshield. I got sprayed with glass as I was putting it in park. I said, ‘Holy s—t, they’re shooting at us!’

MacLellan tried but could not unlock the rifle between the seats. So he put his cruiser in drive, jumped out, and sent his empty vehicle rolling down the road toward the Tsarnaev brothers.

“My thing was, I didn’t know how many guys were there. I didn’t know what type of weapons they had. We weren’t set up. We weren’t ready for this. I needed to create some sort of diversion…they were engaging the vehicle and it was totally empty. It worked out great”.

That’s when the brothers threw the first bomb.

MacLellan: “Officer Reynolds said to me, ‘Sarge, run!’ We got down. The bomb blew up and blew out the windows on that side of [MacLellan’s] cruiser… Thank God, it was a big, high vehicle, too. If you saw the shrapnel on the side of the vehicle, we got very lucky.”.

The brothers threw a second pipe bomb, which bounced a couple times in the street and did not go off. Officer Reynolds radioed for backup.

The brothers threw a third pipe bomb, which also turned out to be a dud.

MacLellan was firing at the brothers from behind a tree, with Reynolds joining in from behind his own cruiser, as three more cops joined the firefight: Officer Miguel Colon, Officer Tim Menton, and Sgt. Jeffrey Pugliese, 57, a married father of two with 33 years on the force.

MacLellan shouted to the suspects that they were surrounded; in response, they threw a fourth pipe bomb. This one detonated and damaged cars and houses, but injured no officers.

“When it exploded it made my eyes shake sideways,” MacLellan said. “It was huge. It was a crazy percussion against your chest.

Sgt. Pugliese, meanwhile, had snuck into someone’s backyard and was working his way toward the brothers from behind a row of houses.

“I could see Tamerlan and Dzhokhar in front of the black Mercedes. I could see muzzle flashes from both sides of the vehicle,” he remembered. “They were moving back and forth, like they were swapping sides… Under the vehicle, I could see shadows and a little bit of their ankles, so I decided to take a few skip shots (shots aimed at the ground that bounce at a low height.) I was trying to take out their ankles. That’s when I believe Tamerlan realized I was there.”

Officer Michael Comick, 25 years old and only four months on the job, pulled up next, still thinking he was only dealing with a carjacking.

Then the rookie heard the fifth explosion. “Just as I’m looking up to see what’s going on, the big pressure cooker went off,” Comick remembered. “That was a very loud bang, and a big glow that lit up the entire night. Car alarms were going off. Things were shattering. I felt like someone dropped a bunch of stones on my head, and it ended up being the shrapnel from the explosion, falling down from the sky.”

“You want more?” the older brother reportedly taunted. “I give you more.”

Sgt. Pugliese braced for battle as Tamerlan rushed him.

He left his position of cover and came running up the street,” Pugliese said. “He was firing at me as he advanced towards me. We were exchanging gunfire… He then had a problem with his firearm — I don’t know if it jammed or ran out of ammunition. He looked down at his pistol in frustration, then he looked back at me. Our eyes met for a moment.”

Tamerlan threw his empty pistol at the Sergeant, hitting him in the left bicep, and then ran. Sgt. Pugliese jumped him and Sgt. MacLellan assisted in trying to cuff the perp.

Just then the Mercedes, with the younger brother at the wheel, began barreling toward them.

“I grabbed Tamerlan by the waist of his pants. I was trying to pull him out of the street,” Pugliese remembered. “Sgt. MacLellan had retreated to a position of cover. Joe [Reynolds] did, too. I had my prisoner; I didn’t want anything to happen to him at this point. The next thing I knew, the headlights were right here in my face, and I had to let go of Tamerlan.

Dzhokhar drove over his brother and dragged him about 25 feet before cutting a corner and speeding off.

Pugliese got up.

“Tamerlan’s still moving, believe it or not,” Pugliese remembered. “It turned out he had been shot nine times, prior to being run over. I sat on him, got his arms behind is back. Joe came up to assist me; together we handcuffed him. He was conscious. He was moaning and still moving.”

With that, the firefight was over.

Miraculously, no cops died in the firefight – not even Richard Donohue, a transit cop, who nearly bled to death after a bullet pierced his femoral artery.

Instead, Donohue would live, and Tamerlan would die.

Himself wounded, the younger brother abandoned the Mercedes a half-mile from the shootout, fleeing on foot, and managing to elude scores of state, local and federal cops until the following evening, when he turned up hiding in a neighbor’s tarp-covered boat.

“I’m wondering, what were they gonna do in New York? Sgt. MacLellan noted of the bombers. “Were they going to go after kids? Police officers? Were they going to some random place, try to kill whoever they could kill and then go out in a blaze of glory?”