An armed gang disguised as police has broken into a jail in northern Mexico and freed more than 50 prisoners.

A convoy of at least 15 vehicles arrived at the prison with gang members all apparently disguised as federal police officers.

Once inside, the prison authorities were persuaded that an inspection was under way and guards were overpowered.

A manhunt is underway for the fugitives - believed to be linked to the powerful drug smuggling Gulf cartel.

Army troops as well as local and federal police have begun a search and set up roadblocks near the prison facility in Zacatecas and neighbouring states.

President Felipe Calderon has declared war on the illegal narcotic organisation in Mexico, deploying 40,000 troops to fight the cartels.

In some regions instances of beheadings and attacks on police have become commonplace, with the deaths of 6,000 people last year alone blamed on illegal narcotics.

But progress in the war against the cartels has been limited by the fact that in the face of well armed and well financed drug cartels, this country's law and order institutions have proved weak and easily corruptible, the BBC's Stephen Gibbs in Mexico City says.