The wannabe terrorists who tried to detonate IEDs outside Gracie Mansion hoped to outdo the 2013 Boston Marathon bombing — and chillingly pledged allegiance to ISIS after their arrests, the feds said Monday.

The disturbing new details were revealed as Manhattan federal prosecutors unveiled a five-count complaint against the two teens from idyllic Philadelphia suburbs — Emir Balat, 18, and Ibrahim Kayumi, 19 — who allegedly plotted to launch a deadly attack Saturday on the Upper East Side.

“This isn’t a religion that just stands when people talk about the blessed name of the prophet,” Balat allegedly told police. “We take action. If I didn’t do it, someone else will come and do it.” 

Investigators asked Balat — a high school senior in Middletown Township, Penn. — if he was familiar with the deadly Boston bombing and whether that was what he and Kayumi were hoping to accomplish with the homemade devices, which were packed with a volatile explosive known as “Mother of Satan.”

“No, even bigger. It was only three deaths,” he replied, according to the 10-page complaint unsealed Monday.

Both suspects expressed allegiance to ISIS in disturbing statements after hero police officers nabbed them during the chaotic anti-Muslim demonstration and counter-protest outside the mayoral residence, the feds said.

Kayumi — a recent high school graduate whose parents own a $2.25 million home in scenic Newtown, Penn. — was captured on police bodycam footage after his arrest, when he responded to someone in the crowd who asked “why he had done this.”

“ISIS,” he answered, according to the complaint. 

He also admitted he watched ISIS propaganda on his phone and that his actions were partially inspired by the international extremist terror group, the charging document states.

Balat, after being taken to an NYPD stationhouse, asked for pen and paper and scribbled a chilling manifesto, the feds said.

“All praise is due to Allah lord of all worlds,” the twisted teen allegedly wrote. “I pledge my allegience [sic] to the Islamic State. Die in your rage yu [sic] kuffar.”

“‘Now die in your rage’ is a commonly known ISIS slogan,” NYPD Commissioner Jessica Tisch said, and Kuffar is Arabic for infidel or non-believer.

“These statements, as set forth in the complaint made clear that this was not random violence. This was a planned attack motivated by extremist ideology and inspired by a violent foreign terrorist organization,” she said, flanked by federal authorities.

“These were ISIS-inspired actions, and it is chilling that they wanted to do something more than the Boston Marathon bombing,” Manhattan US Attorney Jay Clayton said at the joint press conference. 

The two alleged extremists drove to New York City from their idyllic family homes in suburban Philadelphia Saturday morning to carry out the alleged attack, Tisch said.

According to the complaint, Balat first threw one device at protesters during the “Stop the Islamic Takeover of New York City, Stop New York City Public Muslim Prayer” rally organized by right-wing agitator Jake Lang, which had drawn a larger counter-demonstration dubbed “Run Nazis Out of New York City.”

Photos in the complaint show Balat holding one IED in his right hand with the fuse lit, and in another, taken at around 12:15 p.m., he appears to be in the process of hurling it while yelling. 

Immediately after, Balat ran down the block, where Kayumi handed him a second device, which he also apparently ignited and dropped near where several NYPD officers were standing, the complaint alleges. 

One photo in the complaint captured Kayumi looking around moments before he hands the device to Balat, who is reaching for it.

Balat was tackled by cops as he tried to hurdle a police barricade moments later – with one smoking IED seen behind him on the sidewalk, just inches away from an NYPD cop, according to another image.

Photos in the complaint show both IEDs, which consisted of a collection of nails, nuts, and bolts wrapped up in empty containers and wrapped with construction tape.

Fortunately, neither of the homemade bombs exploded at the scene, but were later detonated at the NYPD firing range at Rodman’s Neck in the Bronx, police said.

Tisch praised two of New York’s Finest — Chief Aaron Edwards and Sgt. Luis Navarro — the “courageous and selfless” cops who rushed toward the devices at a moment’s notice. 

“I want to recognize the members of the NYPD who responded on Saturday and in the hours that followed, the officers assigned to that protest who ran toward a lit, improvised explosive device,” the top cop said.

Authorities believe the pair crossed the George Washington Bridge into the city less than an hour before the attack, in a car with New Jersey plates registered to one of Balat’s family members.

Kayumi’s mother filed a missing persons report, claiming he was last seen around 10:30 a.m., the feds said.

The car with Jersey plates was found parked a few blocks from Gracie Mansion and contained more bomb making supplies, the feds said. 

Investigators also found a notebook inside the car containing handwritten notes such as “TATP explosive,” as well as a list of the chemical ingredients “hydrogen peroxide,” “sulfuric acid” and “acetone” and another with bomb parts and quantities including  “aluminum can x6,” and “a box of bolts ect [sic] 2x,” the complaint states.

Sources with knowledge of the investigation told The Post that both suspects had traveled overseas in recent years, including Istanbul and Turkey, known hotspots for ISIS training.

Balat defiantly flashed an ISIS hand symbol as he was led out of a Manhattan precinct and turned over to federal agents on Monday morning.

Balat and Kayumi made their first court appearance before US Magistrate Judge Gary Stain, sporting handcuffs and leg shackles and wearing white hooded jumpsuits. 

Both were mum, staring blankly downward, with Balat repeatedly bobbing his head during the proceedings.

The pair are expected to be placed in protective custody at the federal Metropolitan Detention Center in Brooklyn. They are due back in court on April 8.

Balat’s lawyer, Mehdi Essimidi, was asked by reporters as he left court if he believed his client was a true ISIS disciple.

“I believe he’s 18. He doesn’t have any idea what he’s doing.”

Kayumi’s attorney, public defender Michael Arthus, declined to comment on the case but called his client’s parents “good, decent people.”

— Additional reporting by Amanda Woods