Bahujan Samaj Party legislator Raju Pal was gunned down along with two of his aides in Allahabad at 1500 hours on Tuesday.
His wife and a relative were also seriously injured in the incident.
The legislator's supporters have alleged that Samajwadi Party Member of Parliament Atiq Ahmed is behind the murder.
Pal had defeated Ahmed's brother in a recent assembly by-election.
Since then he had allegedly been receiving threats from Ahmed, who has a long criminal history.
Pal had lodged complaints with the police about two attempts on his life.
However, police records show that Pal too was a history-sheeter with as many as 26 criminal cases against him.
State Home Secretary Pradeep Bhatnagar told media persons in Lucknow on Tuesday: "The MLA was on his way home from the popular Coffee House in Allahabad's Civil Lines when his vehicle came under fire."
"The MLA, who was accompanied by his wife, tried to speed away. The assailants chased his vehicle and gunned him and two of his aides down. His wife and a relative received grievous bullet injuries and were admitted to a local hospital."
The killing brought BSP activists onto the streets on Tuesday.
Meanwhile, the administration has been keeping a strict vigil to avert any communal violence.
"The area is communally sensitive so we are taking all precautions," a senior police official said.