Umesh Pal, the prime witness in the 2005 Raju Pal murder case, was shot dead at his residence here, police said.
One of his gunners, who too was hit in the firing, died later at a hospital, they said.
Prayagraj Police Commissioner Ramit Sharma told reporters that Umesh Pal was attacked with crude bombs and shot outside his house this evening. Two of his gunners too were injured in the attack.
Both gunners were deployed in Pal's security by the government. One of the gunners, Sandeep Nishad, succumbed at the hospital.
Umesh was the prime witness of the 2005 murder of Bahujan Samaj Party MLA Raju Pal. The main accused in the Raju Pal murder is mafia-turned-politician Atiq Ahmed, who is currently lodged in a Gujarat Jail.
A severely injured Umesh Pal was rushed to Swaroop Rani Nehru Hospital where he succumbed during treatment, police said.
Nishad, critical, was put on a ventilator before he succumbed. The other gunner is being operated upon by doctors, police said.
Sharma said that efforts are being made to identify the attackers on the basis of CCTV footage of the area.
"The incident occurred right outside the house of Umesh Pal. So far it has been confirmed that two bombs were thrown and he was fired at by a small firearm," said Sharma.
A case was registered at the Dhoomanganj Police Station on the complaint of the family members of the victim.
"We have formed eight police teams to investigate every aspect of the incident and arrest those behind the attack," the officer added.