Gunmen attacked the Pakistan Stock Exchange building in the city of Karachi on Monday killing two guards and a policeman before security forces killed all four of the attackers, police said.
Separatist insurgents from Balochistan province - the Baloch Liberation Army (BLA) - claimed responsibility in a post on Twitter but Reuters was not able to verify the authenticity of the claim and spokesmen for the group were not available for comment.
"We locked ourselves in our offices," Asad Javed, who works at a brokerage in the stock exchange building, which is in a high security zone that also houses the head offices of several banks, told Reuters.
Javed said he was on the ground floor when he heard gunfire and an explosion and people scattered for safety.
The police chief in Pakistan's biggest city and financial hub, Ghulam Nabi Memon, told Reuters the gunmen attacked with grenades and guns after pulling up in a silver Corolla car.
They threw a grenade at security men posted outside the compound then opened fire on a security post. The four were killed when security forces posted there responded.
Their car was abandoned where they left it.
Two guards and a policeman were killed and seven people were wounded, Deputy Inspector General of Police Sharjil Kharal told media.
A counter-terrorism official told Reuters the attackers were carrying significant quantities of ammunition and grenades in backpacks.
The BLA claimed responsibility in a brief message on a Twitter account set up shortly before the raid, describing it as a "self-sacrificing" attack carried out by its Majeed brigade.
The account was suspended a short time after the attack.
Separatists have been fighting for years in resource-rich Balochistan, complaining the southwestern province's gas and mineral wealth is unfairly exploited by Pakistan's richer, more powerful provinces.
The BLA's Majeed brigade also took responsibility for an attack on the Chinese consulate in Karachi in 2018. Several projects linked to China's Belt and Road initiative are in Balochistan.
This month, three explosions on the same day claimed by a little-known separatist group killed four people including two soldiers in the southern province of Sindh, of which Karachi is capital.
The Pakistan Stock Exchange PAKS.PSX did not suspend trading during the attack. Its main KSE-100 index dropped 220 points but it later recovered and was 200 points higher at 0830 GMT.
Islamist militants have also launched attacks in Karachi and elsewhere in Pakistan over the years but their violence has become less frequent after military operations against various factions in strongholds on the Afghan border.