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.
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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 DGP OP 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."
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."
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."