KARACHI, PAKISTAN - Pakistani police say an unknown number of gunmen have attacked the Pakistan Stock Exchange in the southern port city of Karachi, the country's financial center.

Rizwan Ahmend, a police official at the scene of the attack says gunmen opened fire at the entrance and have entered the stock exchange grounds. It wasn't immediately known how many gunmen are involved and whether they have been able to enter the building itself in Monday's attack.

Inside the stock exchange, broker Yaqub Memon says he and others are huddled inside their offices.

Heavily armed special forces have surrounded the building located in the heart of the city's financial district, where the Pakistan State Bank is located as well as the headquarters of several national and international financial institutions.

Local television stations are showing police in full body armor surrounding the building but still staying outside the high walled compound of the stock exchange.