Initial reports suggest a group of four attackers have stormed the building, after throwing a grenade and have fired several shots.
There are reports of at least two people killed, however, the number of casualties is not yet verified.
The incident is believed to be ongoing; security forces have reportedly cordoned off the area.
As per reports, people from inside the building are being evacuated.
Faisal Edhi, the chief of the country's main rescue ambulance service, said four attackers have been killed by the security forces so far.
"Two were killed at the gate, and two of them went inside in an injured state and were then killed there," Edhi said, speaking to reporters close to the site of the attack in Karachi's main business district.
Further details are awaited.
The stock exchange also confirmed the attack in a separate statement.
The Karachi stock exchange is Pakistan’s largest and oldest stock exchange, incorporated with the exchanges in Islamabad and Lahore.
MNA/Agencies
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