He was rushed to the trauma centre where he succumbed to his injuries.
According to reports, the attackers, wearing saffron clothes, walked into Tiwari's office in Khurshid Bagh area on the pretext of handing over a box of sweets.
Soon after walking in, the assailants opened the box, took out the firearm and sprayed Tiwari with bullets and fled.
Tiwari was taken to the trauma centre but could not be saved.
Tiwari, a former Hindu Mahasabha leader, had formed the Hindu Samaj Party in January 2017. Tiwari had earlier served as the president of the Hindu Mahasabha.
He had made highly controversial remarks involving Prophet Muhammad. He was arrested and the National Security Act (NSA) was slapped on him. Recently, the Lucknow bench of the Allahabad High Court revoked the NSA against him.
Lucknow Senior Superintendent of Police Kalanidhi Naithani said police teams have been formed to arrest the assailants.
This is the fourth murder of a right wing leader this month.
On October 8, BJP leader Chaudhary Yashpal Singh was gunned down in a similar manner in Deoband.
On October 10, a BJP leader Kabir Tiwari, also a former student leader, was shot dead in Basti which led to student groups going on rampage and burning down government vehicles.
On October 13, BJP corporator Dhara Singh, 47, was gunned down by unidentified assailants in Deoband in Saharanpur on Saturday morning.
( With inputs from IANS )