Most of the inmates at a youth prison in Brazil's biggest city attempted to escape during a mass jail break before dawn Wednesday, authorities said.

Virtually all of the 99 inmates held at the jail in a poor neighborhood tried to flee the fenced Vila Maria detention center and 48 were still at large by early afternoon, said Denilson Araujo, a spokesman for Sao Paulo's youth prison system.

Araujo said he did not know how the inmates got out of the jail's fenced compound about 5:30 a.m., but Globo TV reported that authorities are investigating the possibility that the inmates took a guard or guards hostage as part of the escape.

Authorities captured 36 inmates as they were attempting to escape, but it was not immediately clear whether they were inside or outside of the compound when they were detained.

Prison riots are common in Brazil, frequently sparked after authorities quell escape attempts, though actual escapes are less common.

Human rights groups have condemned conditions in Brazilian prisons for years, saying they are frequently overcrowded with horrendous inmate conditions.

Vila Maria jail, however, held 99 inmates and had a capacity for 100 prisoners between the ages of 17 and 20.