At least five people have died after shots were fired at the Pakistan Stock Exchange in Karachi, according to rescue officials on Monday.
Policemen and security officials are among those confirmed dead, according to Mohammad Azeem, an official with the Edhi rescue center, a prominent local charity service.
Rescue and paramilitary security personnel are at the scene in an ongoing operation, Azeem said.
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 in a media briefing on Geo News that four attackers entered the compound and started firing but were killed by security forces.
"There were four attackers who came from the parking lot, they threw a grenade at the main entrance of the KSE (stock exchange) and then started firing indiscriminately," Ali said.
"The attackers were wearing uniform that looked like police uniforms, all four have been killed and the situation is currently under control."
The Balochistan Liberation Army (BLA) group has claimed responsibility for the attack, according to Maj. Gen. Omar Ahmed Bukhari, the director general of the Pakistan Rangers Sindh, a paramilitary force under command of the country's military.
"We are investigating further, to find out who these people are," Bukhari said.
A BLA source, confirming the claim, said: "Majeed Brigade of Baloch Liberation Army today has carried out a self-sacrificing attack on Karachi stock exchange. Our fighters are inside the building and have taken control of the area."
The BLA has conducted a low-level separatist insurgency against targets in Pakistan for nearly two decades. The group was initially formed due to anger over the state's perceived monopoly over Balochistan province's mineral resources.
It has previously killed and kidnapped Chinese engineers working in the province.
A tweet from the official Twitter page of the Pakistan Stock Exchange said, "The situation is still unfolding and management, with the help of security forces, is managing the security and controlling the situation," it said.
"We will issue a more detailed statement in due course once the situation is under control and more details are known. For now firing has stopped and reinforcement are in place," the tweet read.
The Pakistan Stock Exchange administration sent out a message saying that "rangers" had entered the compound and that "four terrorists" were dead.
The administration told workers inside the building to stay on the higher floors and that "the rangers will comb the compound and then let everyone leave in an orderly fashion."
The Pakistan Stock Exchange in Karachi is the country's largest. It's located in the city's financial hub, where all major banks and financial services are located and where there is usually a heavy security presence.