Diehard Luigi Mangione fans rally outside NYC courthouse as he pleads not guilty to murdering UnitedHealthcare CEO Brian Thompson in federal death penalty case

Diehard Luigi Mangione fans protested the feds’ death penalty case against their sick hero Friday — as he pleaded not guilty to the cold-blooded killing of UnitedHealthcare CEO Brian Thompson.

Mangione, 26, stood in tan jail garb when he was asked to enter his plea in Manhattan federal court to a four-count indictment charging him with murder, stalking and a firearms offense for the shocking Dec. 4 hit on the health insurance bigwig.

“Not guilty,” he said,

The plea kicks off a fight for Mangione’s life, as the feds have said they will seek the death penalty — much to the chagrin of roughly 20 fans flamboyantly supporting him outside.

“Luigi is a fall guy,” declared April Smith, 49, who trekked from Long Island City wearing gold eyelashes, a yellow- and red-sequined shirt and fluffy Uggs to support Mangione.

Smith said she was motivated to attend her first ever court case after hearing Mangione could be sentenced to death, if convicted.

Her father was a corrections officer at Joliet Correctional Center, Illinois, of “The Blues Brothers” and “Prison Break” fame, and told her horror stories of inmates being mistreated on death row.

“The death penalty is inhumane,” she said. “I am hoping he will find the right people to help him out of his situation.”

Many protesters’ signs seen outside the courthouse also held the anti-health insurance company slogan, “Deny, Delay, Depose.”

The use of the three-word slogan has skyrocketed in popularity after it was reported that the ominous message was scribbled into ammunition recovered at the Manhattan murder scene.

His Friday arraignment was Mangione’s first appearance in federal court since US Attorney General Pam Bondi revealed that she’d take the rare step of moving for capital punishment in the shocking slaying.

In a notice of intent to seek the death penalty filed Thursday, prosecutors cited Mangione’s alleged “intent to provoke broad-based resistance to the victim’s industry.”

The feds seek the death penalty against Mangione, seemingly in part because of the cult following his arrest — which has translated to a $1 million legal defense fund.

New York state outlawed capital punishment in 2004. But federal prosecutors can still execute defendants — if they convince a jury to unanimously sign off on a government-sanctioned killing.

If they did so in the Mangione case, it would be the first Manhattan federal execution in 70 years.

The Maryland native is separately charged in Manhattan state court with murder as an act of terrorism and other charges that carry a possible life sentence without parole.

The state case is currently on pace to head to trial first.