The attackers pummeled the woman's face and tied up her male companion, a French citizen, then beat him with a metal bar as he witnessed the harrowing assault. The couple were forced to use bank cards to withdraw money from their accounts before the assailants finally freed them at a bus station on the outskirts of Rio de Janeiro.
The assault stunned many in Brazil, especially as Rio tries to promote itself as a city on the mend and prepares to host the 2014 World Cup and 2016 Olympic Games.
"Everyone should be shocked by this horrendous crime," said Aparecida Goncalves, the head of Brazil's national office for combating violence against women. She said that reports of gang rape remained relatively rare in Rio and other parts of Brazil, but that cases of rape on public transportation including buses and subway cars remained a pressing issue in some large cities.
Two men were arrested over the weekend, one of whom, the police said, confessed to the rape of the 21-year-old woman. The police said she had been in Brazil on a student visa. A third suspect was arrested Monday night.
In addition to setting off calls for better policing, the assault led to comparisons in the Brazilian news media to recent episodes in India, including the fatal beating and rape in December of a 23-year-old student on a moving bus in New Delhi, and the gang rape of a Swiss tourist in March in central India.
The number of female tourists to India has recently fallen more than 30 percent as fears over sex crimes in the country persist, the Associated Chambers of Commerce and Industry of India said Monday.
The assault on the American woman here, police investigators said, began early Saturday, just after midnight, when the attackers forced other passengers out of the van, which picks up passengers along the street and can seat about a dozen people. The woman was raped by all three men, who took turns driving the vehicle, the police said.
"It was a gang rape," said Jayme da Costa Rosa Neto, a police official investigating the attack.
After the couple were freed about 6 a.m. Saturday and left at a bus station, the woman was taken to two public hospitals, Miguel Couto and Rocha Maia, for treatment including the administering of a cocktail of drugs containing the morning-after pill, to prevent pregnancy, and other medications to prevent sexually transmitted diseases.
The police said the victim had severe swelling around her nose, and that her companion, 23, had been hit in the area around one of his eyes. The woman left Brazil after registering the crime and undergoing preliminary medical treatment, while her companion remained here, where he is cooperating with the police, said Alexandre Braga, a senior police investigator with Rio's special police unit for crimes involving tourists.
The two men in custody were arrested after investigators tracked purchases made with the victims' credit cards, which were stolen by the assailants, and examining images obtained from security cameras at a filling station and convenience store where the men had stopped to buy energy drinks and whiskey.
After news of the arrests was broadcast Sunday night on Fantastico, a widely viewed news program on the Globo television network, other people here came forward to tell the police they recognized the assailants in connection with other crimes, largely robberies, aboard what appeared to be the same transport van.
One 21-year-old Brazilian student said she had similarly been held for an hour and raped by the same men on March 23, after boarding the van.
The revelation of that previous episode seemed to have shaken the public security forces here. The victim had quickly registered the case with the police, but the authorities were said to have slowly investigated the claim.
Two police officials in charge of investigating the March 23 case were abruptly removed from their posts Monday.
Brazil has recently grappled with other high-profile cases of gang rape, including one episode last year in Queimadas, a city in the northeast Paraiba state, in which six men were convicted of raping five women at a birthday party. Two of the women were killed after recognizing their attackers.
More broadly, reports of rape in Brazil have climbed significantly since 2009, when the nation's criminal code was changed to expand the legal definition of rape to include crimes involving anal penetration. More than 5,300 people, about 90 percent of whom are women, registered cases of rape in the first six months of 2012, an increase of more than 150 percent since 2009.
Goncalves, the federal official in charge of combating violence against women, said much of the increase in reports of rape involved efforts to encourage victims to report the crimes. "Women are more courageous about coming forward with what happened to them than in the past," she said.