Police and security officials are among the confirmed dead, according to Mohammad Azeem, an official at the Edhi rescue center, a leading local charity service.
Paramilitary rescue and security personnel are on site in an ongoing operation, Azeem said.
The video shows security and barricades outside the stock exchange in Karachi, according to CNN affiliate Geo News.
Abid Ali, director of the Pakistan Stock Exchange, said at a press conference on Geo News that four attackers entered the complex and started shooting, but were killed by security forces.
"There were four attackers who came from the parking lot, threw a grenade at the main entrance of the KSE (stock exchange) and then started firing indiscriminately," said Ali.
"The attackers were wearing uniforms that looked like police uniforms, all four were killed and the situation is currently under control."
The Balochistan Liberation Army (BLA) group claimed responsibility for the attack, according to Maj. Gen. Omar Ahmed Bukhari, chief executive officer of the Pakistan Sindh Rangers, a paramilitary force under the command of the country's army.
"We are investigating further, to find out who these people are," Bukhari said.
A BLA source, confirming the claim, said: "The Baluchis Liberation Army Majeed Brigade has carried out a slaughter attack on the Karachi stock exchange. Our fighters are inside the building and have taken control of the area." .
The BLA has carried out a low-level separatist insurgency against targets in Pakistan for almost two decades. The group was initially formed out of anger at the state's perceived monopoly over the mineral resources of the province of Balochistan.
He has previously killed and kidnapped Chinese engineers working in the province.
"We will issue a more detailed statement in due course once the situation is under control and more details are known. For now, the shooting has stopped and the reinforcement is in place," the tweet said.
The administration of the Pakistan Stock Exchange sent a message saying that "rangers" had entered the complex and that "four terrorists" were dead.
Management told workers inside the building to stay on the top floors and that "rangers will comb the complex and then let everyone leave in an orderly manner."
The Pakistan Stock Exchange in Karachi is the largest in the country. It is located in the financial center of the city, where all the main banks and financial services are located and where there is generally a strong security presence.