Saurashtrajeet Singh had seen the two assailants, Moinuddin and Ashfaq, and had ushered them into Tiwari's office.
Singh has been associated with the Hindu Samaj Party since Tiwari floated the outfit after quitting Hindu Mahasabha.
Saurashtrajeet Singh is now living in his village in Kasia in Kushinagar.
"The two assailants were masquerading as Hindus and had come to seek Tiwari's help in marrying a Muslim girl.
"One of them claimed to be in love with a Muslim girl. I heard Tiwari promising legal and other help for the wedding. Then I left the place," he told the local reporters.
He further said: "I can recognise the two who killed Kamlesh Tiwari and now I am the sole witness and also the prime target. But Uttar Pradesh police has not paid heed to my pleas for security."
Singh said that if the assailants could come from Gujarat and easily kill Tiwari, then he was a sitting duck. "I have not stepped out of my house since the day of the murder. I thought the police would give me the security, but they have dumped me," he said.
Recalling the event on the day of the murder, Singh said: "I clearly remember the face of the man with the cap, who gave me Rs 100 to buy a packet of cigarettes. I returned within five minutes with the cigarette packet, but then Bhaiyaji (Tiwari) asked me to get 'pan masala'. When I returned, Bhaiyaji was lying in a pool of blood and I saw the duo running away. I have been with Bhaiyaji for three years, but had never seen or heard about them."
Singh is a coordinator with the Hindu Samaj Party in Kushinagar and is staying with his father, Rajeshwar Singh and elder brother, Swatantrajeet Singh.
( With inputs from IANS )