Suspected members of a Mexican drug cartel disguised as federal police entered a state prison in the northern state of Zacatecas on Saturday and freed 59 inmates.
Most of the prisoners who escaped are believed to be members of the armed wing of Mexico's Gulf Cartel, one of the country's most powerful criminal organisations.
Officials say the early morning jailbreak was carried out by heavily armed men who arrived in 15 vehicles and a helicopter.
They gained entry into the prison after claiming they were there for a prisoner transfer.
Mexican President Felipe Calderon has staked his presidency on crushing drug gangs that killed 6,300 people last year.
The United States is increasingly alarmed by the violence, fearing it could spill across the border.
