The militants launched a grenade attack at the main gate of the building and attempted to storm it after opening indiscriminate fire.
At least two civilians were killed in the attack and multiple were injured. The surrounding areas have been sealed by police and Rangers personnel.
The injured are being shifted to a nearby hospital for medical treatment. People inside the PSX building are being evacuated from the back door.
Weapons and hand grenades have been recovered from the attackers.
According to rescue services the number of casualties may increase.
Speaking to local media, the Inspector-General Karachi police said that the situation was under control and all militants had been killed in the attack. He said that Rangers and police officials had entered the building and were conducting a search operation.
According to the IG, the attackers were reportedly wearing clothes usually worn by police officials while they are off-duty.
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