A Rome university student was burned alive by her ex-boyfriend after she left him, authorities said.
One investigator called it the most atrocious crime he has seen in his career.
Sara Di Pietrantonio, 22, died after Vincenzo Paduano, 27, set her car on fire, prosecutor Maria Monteleone told reporters.
He then chased her after she ran out of the car, setting her ablaze when he caught up with her, authorities said.
Investigators said her attacker used a cigarette lighter to set Di Pietrantonio's face on fire after dousing her with alcohol.
"I can say that in 25 years in this work I have never seen something so atrocious," said Luigi Silipo, the lead police official in the investigation.
Paduano was being held for investigation of premeditated murder, Monteleone said.
The suspect first denied killing the woman, but after eight hours of interrogation, confessed to killing her, Silipo told reporters.
Paduano "didn't accept being abandoned' by the woman, Monteleone said. 'He organised, he planned the aggression,' the prosecutor said.
Silipo said the suspect walked off his job as a security guard about 3 am local time and waited outside the home of Di Pietrantonio's current boyfriend.
Then, after the woman left the home and drove off by herself, Paduano drove off, eventually forcing her car to the side of the road, he said.
"He got into her car, and after an argument, doused the car (interior) with a small bottle of alcohol, and doused Sara, too," Silipo said.
"She ran out, he torched the car, caught up with her, and after about 100m" set her ablaze, leaving her to die "in an atrocious manner," the police official said.
A surveillance camera in the area captured some of the events, including at least two cars that passed by while the woman screamed in vain for help as she tried to flee, authorities said.
Monteleone made what she said was a fervent appeal to citizens to help such women, "not to look the other way". She added that if passers-by had helped, the woman's life might have been saved.
She also encouraged women "not to keep hidden any threatening behaviour by those who insist they love you, but it's not that way."
Italian women's advocates have been trying to change mentalities in a country where men often turn violent when a women breaks off a relationship.
One such champion for women to be more assertive in protecting themselves is an Italian lawyer whose face was mutilated in an acid attack ordered by her ex-boyfriend. She courageously testified at the ex-boyfriend's trial.
Sounding a call today to Italian woman to denounce threats by men to police was Chamber of Deputies President Laura Boldrini.
The parliamentary leader said a change in cultural mentality was needed, starting in early childhood classrooms. Women "must understand that those who should be ashamed are the violent ones, not the women who suffer threats," Boldrini said.
