A biker gang so vicious that one member removed his own artificial leg to beat a man senseless with the limb is aggressively expanding its turf in the Big Apple, warns a former Bureau of Alcohol, Tobacco, Firearms and Explosives agent who infiltrated the group.

“Pagans are riding the streets of New York City today, flying their colors,” ex-ATF agent Ken Croke, 54, who spent two years undercover with the gang, told The Post. “There are s–tloads of Pagans running around, actively recruiting and have been for years.”

He chronicles his harrowing covert experiences a decade ago in the new book, “Riding with Evil: Taking Down the Notorious Pagan Motorcycle Gang,” co-authored with Dave Wedge.

Twenty Pagans were arrested and pleaded guilty to racketeering, drugs, firearms and conspiracy to commit murder, among other charges, following his undercover investigation. The gang has stormed back in recent years however, the former fed says.

Croke was an ATF supervisor in Boston in 2008 with a history of undercover operations, including efforts to take down the Vagos and Mongols motorcycle gangs in Los Angeles, when a series of events put him deep inside the Pagans of New York as they seized more northeast turf.

“Pagan members sold crystal meth and guns, gang-raped women, brutalized rivals, and extorted businesses,” he writes.

Croke witnessed the gang hitting up local businesses for old-fashioned protection money; watched in shock as an aging Pagan named Tumbleweed — who “looked like Blue from the Will Ferrel movie ‘Old School’” — removed his wooden leg and beat a man with it outside a Long Island barroom; and spent time behind bars to maintain his cover after being arrested with a handgun.

He even helped Pagans move the body of an apparent murder victim from one upstate grave to another. Unearthing the corpse near Swan Lake, 80 miles north of New York City, led to a vigorous debate on law-enforcement ethics within the ATF.

“The thought was that the guy was dead and would not be any less dead a few months later,” Croke writes.

Croke often slept on the floor of a fetid home that was more sewer than crusty gang hang in Middle Island, Long Island, “surrounded by the filth and the scent of stale beer, weed and cigarettes,” he recalls, among Pagans who “smelled like s–t and snored like bears.”

A Pagan known as Hogman “laid out a plan to brutally rape” a female acquaintance one night while he was drinking beers and snorting cocaine in their filthy hideaway, Croke writes. He risked his cover to warn “Tiffany” of the plan. She fled, never to be seen again.

Recruits are drilled in gang history and lore, Croke says. The Pagans were founded by legendary biker Lou Dobkin in Maryland around 1957. They wear patches with Pagans in blue medieval-style font across a white “cloud,” with red stitching around the edges.

The cloud patch is accompanied by the image of sword-carrying Norse god Surtr, plus a 1% patch meant to denote the most violent motorcycle gangsters. Affiliate Pagans sport a blue-on-white number 16 in the same medieval font. P is the 16th letter of the alphabet.

The white cloud was originally meant to indicate white supremacy, Croke says, and the red trim the bloodshed by gang members to defend their turf and their values. The gang in recent years has shelved much of its white supremacy rhetoric in the interest of expansion. Members in NYC now are often Hispanic.

Pagans’ presence in the city expanded following a hostile takeover of the gang by Keith “Conan” Richter of Bay Shore, Long Island, in 2018. The ruthless outlaw spent 16 years in prison for ordering the murder of a Long Island strip club owner who refused to pay the Pagans extortion money in 1998. He was convicted of recent weapons charges in October and is currently serving a 36-month sentence.

Pagans today number about 2,000 members nationally with 12 chapters in New York, claims Croke: four on Long Island, four upstate and four in New York City. The Bronx has historically been Pagans home turf within the five boroughs.

The archrival Hells Angels started to squeeze their Bronx territory when they fled their infamous clubhouse at 77 East 3rd St. in the East Village in late 2019 for an abandoned American Legion hall on Longstreet Avenue in Throggs Neck. The Pagans quickly responded, firing up to 14 gunshots into their rival’s new digs in early January 2020.

Francisco Rosado, the leader of the Bronx chapter of the Pagans, whose face was completely covered in prison-style tattoos, was gunned down on Holland Avenue in broad daylight just a few months later in May 2020.

His two assassins wore masks and carried handguns fitted with long silencers. Hells Angels honcho Frank Tatulli, 58, and club member Sayanon Thongthwath, 29, were arrested two months later and charged with the murder.

The Pagans are the more dangerous of the two notorious gangs, claims Croke.

“The Hells Angels, in my opinion, they’re like a business,” he said. “They have copyrights and doctors and lawyers who are members not involved in criminal activity. The Pagans are just violent individuals. They’re bad dudes. They don’t have two nickels to rub together but they don’t care.”

Hells Angels “sleep in five-star hotels,” he said. “Pagans sleep in dirt fields.”

The gang’s recent public resume includes the baseball-bat beating of a Hells Angel by Pagan Robert “Hellboy” DeRonde at a Newark gas station in 2018 and a shooting on the New Jersey Turnpike in 2020 that led to the indictment this past December of “high-ranking” Pagans Larry “Savage” Ortiz and Junius “Jayo” Aquino.

The gang has a diverse criminal operation. Pagans are “engaged in criminal activities such as arson, assault, bombing, extortion and murder,” states the DOJ in a 2021 report on outlaw motorcycle gangs.

“The Pagans intimidate, the have a history of white supremacy, they’re misogynists, and they pump communities full of drugs and misery,” said Wedge, the book’s co-author. “They’re outlaws and some of them are downright sub-human.”