By Nathan Morley
According to reports, some staff inside barricaded themselves in offices in the heart of Karachi's financial district, whilst others were swiftly evacuated. Soon after, security operatives fought back, killing all four attackers.
According to local media, two security guards and a policeman were killed.
The Karachi stock exchange is the nation’s main financial centre, and incorporates exchanges in Islamabad and Lahore.
Pakistan has suffered years of militant violence, mostly by Islamist groups.
Members of the Baloch Liberation Army say they are responsible for the atrocity.
The group is fighting for a separate homeland and more resources in Pakistan's Balochistan province.