Hasan, who is the leader of the Opposition in the Sindh Assembly, was going home after offering Eid-ul-Azhar prayers when four unidentified gunmen on two motorcycles, wearing police uniforms, ambushed his vehicle in the Buffer Zone area of the southern metropolis, they said.
While Hassan remained unharmed in the incident, an attacker was killed during the retaliatory fire by one of the guards of the Muttahida Qaumi Movement-Pakistan (MQM-P) lawmaker.
Assailants on three motorcycles had attacked the politician after he left a mosque in Buffer Zone for home after Eidul Azha prayers, police said.
Five people have been wounded in the attack, they said.
MQM-P leader Faisal Sabzwari tweeted that the attackers were in police uniforms and during the gun attack, Hasan's three guards got injured while one, a police constable, was killed. A teenager was also killed in the attack.
One of the assailants was shot dead in retaliatory firing, police said, while another was injured but managed to flee.
Police also said they had recovered a 9mm pistol and a motorcycle used by the killed attacker from at the scene of the incident, the report said.
Prime Minister Shahid Khaqan Abbasi condemned the assassination attempt on Hassan and summoned a report of the incident from concerned authorities.
Meanwhile, Interior Minister Sindh Anwar Syal stated that complete security has been given to Hasan and he has personally spoken to him.
MQM is the fourth largest political party in Pakistan. It represents the Urdu speaking population in Sindh province.