About 40 buses blocked one of the main highways leading north out of Mexico City in a protest over the latest killing aboard a bus by thieves.

Bus routes around Mexico City have become a favored hunting ground for bands of thieves who board buses, draw weapons and rob passengers before fleeing. In the latest incident, two thieves fatally stabbed a bus driver's assistant when he resisted their demands. The killers escaped.

Bus drivers organized Friday's blockade to demand authorities do more to ensure the safety of commuter bus lines.

Such mass robberies occur on almost a daily basis, but attackers in the past have generally not harmed drivers, although they have routinely killed passengers who resist.