The legless Olympian dubbed the “Blade Runner” gunned down his model girlfriend as the two argued furiously in his South African home, authorities charged yesterday.

Oscar Pistorius — who warmed hearts around the world when he competed at the London Games on carbon-fiber legs — was charged yesterday with murdering Reeva Steenkamp, 29, at around 4 a.m.

The cover girl, who was both the country’s top model and a brilliant businesswoman with a law degree, was shot four times with a 9mm handgun. She was struck in the leg, chest and head.

She had been dating the 26-year-old sprinter, a gun enthusiast, for only a few months — and she may not have been fully aware of his gun-loving ways.

Pistorius always kept a loaded pistol next to his bed, a machine gun by his window and cricket and baseball bats behind the door when he slept — and once bragged on Twitter about his skill with “headshots.”

He also wrote about how he nearly opened fire on one of his home appliances.

“Nothing like getting home to hear the washing machine on and thinking it’s an intruder to go into full combat recon mode into the pantry!” he tweeted in November.

South Africa has one of the world’s highest violent-crime rates — nearly 50 people are killed daily in the nation of 50 million — but Pistorius bragged he was well-prepared to defend himself.

“Had a 96% headshot over 300m from 50 shots! Bam!” he tweeted in November 2011.

He also told a New York Times Magazine reporter that a night before their interview he was alarmed by a noise and investigated it with gun in hand.

He then dragged the writer to a shooting range.

He said he felt safe in his guarded complex but kept an eye on the security officers.

“The problem is when the guards are in on the crime. It’s usually safe in guarded estates like this until that happens,” he told the Daily Mail last year.

Just before Steenkamp was slain, the couple, who did not live together, had a knock-down, drag-out fight, neighbors said.

“We have received some reports about screaming and shouting. This is part of an ongoing investigation that is far from finalized,” said Brig. Denise Beukes, a South African police spokeswoman.

Authorities said the two were the only ones home in the gated community and that there was no sign of forced entry.

There was an earlier incident of domestic violence at the luxe Pretoria digs, but cops would not say whether it involved Steenkamp, a former FHM model and star of the reality-TV series “Tropika Island of Treasure.”

Local media suggested Pistorius may have mistaken her for a burglar after she tried to surprise him for Valentine’s Day, but cops shot down that theory.

“You cannot be charged with murder if it’s a case of mistaken identity,” Beukes said.

Sources told The Post the double-amputee sprinter clammed up after he was busted.

Wearing a gray hoodie with his hands shackled, he walked on standard prosthetic legs, unlike the carbon-fiber blades with which he competed at the London Olympics.

His family delivered blankets and a home-cooked meal to make his overnight stay in a cell more comfy. He was due today in court, where authorities are expected to argue against bail.

His lawyer, Kenny Oldwage, told SABC TV that Pistorius “is doing well but very emotional.”

Investigators roped off Pistorius’ Silver Woods Estate home yesterday, grilling neighbors and searching for evidence.

The community is surrounded by electric fences and is patrolled by armed guards.

They recovered a 9mm handgun owned by Pistorius, who at one point was taken under heavy guard from the police station to a hospital for a medical checkup.

His arrest could prove costly.

Pistorius was paid up to $6 million a year for endorsement deals and appearances. His sponsors include US companies Oakley and Nike.

Nike yesterday pulled an online ad in which he was shown running while on-screen text read, “I am the bullet in the chamber.”

Pistorius — whose legs were amputated before his first birthday because of a birth defect — is no stranger to violence.

He was busted for allegedly slamming a door on a woman in his home in 2009 — charges that were dropped — and threatened to “break the legs” of a man he believed had slept with a love interest.

Steenkamp, meanwhile, was both beautiful and brainy — graduating from law school before becoming a model for cosmetics giant Avon in South Africa.

Family and friends remembered her as a kindhearted woman with a zest for life — and said she never mentioned having problems with Pistorius, who started dating her two months ago.

“It was such a devastating shock that her whole life, what she could achieve, never came to fulfillment,” said her uncle, Mike Steenkamp.

Eleni Symeonides, 30, a Reeva friend from South Africa, recalled their last chat.

“We were joking about things girls joke about,” said the actress, now living in West Hollywood.

Mike Steenkamp added, “The family at the moment . . . are devastated.”

Additional reporting by David K. Li and Post Wire Services