Police were searching for a third suspect in the rape of an American student who was violated as her French boyfriend looked on during a horrific six-hour abduction aboard a Rio minibus.

The crime raised fears ahead of World Youth Day in July, when the city is set to welcome Pope Francis and some 2.5 million young people. Brazil hosts the 2014 World Cup and 2016 Summer Olympics.

Two male suspects, age 20 and 22, were detained over the weekend, and another is still at large, a police spokesman said.

"She is a US national and he is French; they were studying in Rio," a French consular source told AFP, in reference to the victims.

The American, 21, and the Frenchman, 23, according to local media, boarded a minibus around midnight Saturday in touristy Copacabana headed to Lapa, a trendy area home to popular bars and dance clubs.

Two men who also boarded the minibus ordered the rest of the passengers to get off and handcuffed their victims.

They then proceeded to beat the young man with a metal bar and rape the young woman as they rode around the city, the special police for aid to tourists (DEAT) said.

The driver of the minibus may also have taken part in the rape, local media reported.