Elsewhere in the city, Professor Jose Alfonso Martinez, a member of the Social Sciences Institute of the Autonomous University of Ciudad Juarez, was shot in front of his wife in an attack by at least four men in a residential area.

His wife was unhurt.

Martinez is the third professor at the Autonomous University to be killed this year, with a fourth from the Ciudad Juarez branch of the University of Chihuahua.

Another nine men were killed in six separate incidents.

Hospital raid

Also on Friday, assailants entered the waiting area of an emergency room, causing panic. The Mexican military surrounded the clinic, but no arrests were made.

Armed men have entered hospitals in the past to try to kill victims who have survived earlier shootings.

Mexico has strict gun-control laws, prohibiting the purchase of assault rifles and requiring gun purchases to be registered with the government.

But gun-control advocates say the US plays a large role in gun violence as 90 per cent of the weapons confiscated in Mexico have been either purchased in the US or smuggled in from there.

Barack Obama, the US president, has promised a crackdown on guns from the US.