Seven people riding motorcycles have hurled grenades at a Karachi school, resulting in a lock-down on schools in the area.
Students and staff were not present during the incident and no casualties have been reported so far in Karachi's Gulshan-e-Iqbal neighbourhood.
Before fleeing the scene, the perpetrators left a letter behind, authorities said. Schools and other academic institutions in the area have been asked to suspend operations.
The grenade attack is thought to be in retaliation after Pakistan hanged two terror convicts affiliated with the extremist group Laskhkar-i-Jhangvi in the central prison in Karachi.
Authorities said the attackers aimed to create panic among children and parents as well as the general public in the area in the wake of the deadly massacre at Peshawar Army School.
Provincial education minister Nisar Ahmed Khuhro has ordered an investigation into the attack, and top police officials are at the scene.
After the Peshawar attack Pakistani authorities have introduced a range of security measures especially for educational institutions, including allowing the teachers to carry weapons to school.