The president of a little known Hindu Samaj Party was murdered at his home in the congested Naka Hindola area of the city here, police said on Friday.
Kamlesh Tiwari, 45, was earlier associated with a faction of the Hindu Mahasabha, they said.
The Akhil Bharatiya Hindu Mahasabha in a statement condemned the incident. Police said brief protests were held in the area by local residents after the murder.
"Kamlesh Tiwari was found murdered inside his house. Two persons had come to meet him. Both were speaking to Tiwari when the latter sent one of his aides to get 'paan' for them," Vikas Tripathi, ASP (Lucknow West) said.
"When his aide returned from the market, he found Tiwari in a pool of blood," Tripathi added.
A senior UP government official said the visitors shot Tiwari dead after talking to him for little over half an hour. Police reached the spot and gathered the CCTV footage.
UP Director General of Police O P Singh told reporters, "Those who have murdered Tiwari are said to be his acquaintances. They spent over half an hour with him and even gave Tiwari a box of sweets and after it the murder took place. We have got some vital clues and based on CCTV footage and other evidences, we are working on the case."
He said teams had been formed to crack the case.
"We have also got some call details. The special task force has been engaged in the case," he said, adding that it was "purely a criminal activity" done by two people.
The UP DGP said there were "some minor law and order problem" in the area after the murder took place with local people saying that such incidents should not take place.
"But, the situation now is under control. Police officers are patrolling the area. In the next 48 hours, we will catch the culprits," he said.
When asked whether there was any possible terror angle in the incident, Additional Director General of Police (Law and Order) PV Ramasastry told PTI, "All the angles are being looked into. And, it would be improper to jump to any conclusion."
Senior Superintendent of Police Kalanidhi Naithani said prima facie it seemed to be a case of personal enmity.
He added that teams had been formed to crack the case.
Naka Hindola SHO Sujeet Kumar Dubey said there were brief protests by angry local residents after the incident. The situation was normal, Dubey said.
An eyewitness, Saurashtra Singh, told reporters shortly after the incident that the police constable deployed at the house of Tiwari was sleeping while the gunner attached to him was not coming for the past a couple of days.
Tiwari's wife Kiran in a complaint to police said, "Around 12.30 pm, my husband was sitting in his office located on the first floor of the house and speaking to some unknown persons. I was in another room. Suddenly, for nearly 4-5 minutes, there was no conversation.
"I came out to enquire and saw my husband lying in a pool of blood. The two persons, who had come to meet him, were running out of the office."
She further alleged, "In 2016, Mohammed Mufti Naeem Kazmi and Imam Maulana Anuwarul Haq, both from Bijnor district, had announced a cash reward of Rs 1.5 crore on my husband's head. They have killed my husband by hatching a conspiracy."
Meanwhile, the Akhil Bharatiya Hindu Mahasabha in a statement termed the murder of Tiwari a shameful incident.
The UP chief of the mahasabha, Piyush Kant Verma, said the killers should given capital punishment. He also urged the Centre to provide security to all leaders of the Hindu Mahasabha.