Police said Tunda was running two madrasas in Karachi under the name of Mehdud-Taleem-Islam-E-Darul-Funoon, for which he got a lot of donations from individuals and terror organisations.
He was also teaching in several other madrasas in Karachi to indoctrinate youths and children to join jehad.
"Apart from giving motivating speeches, Tunda was training these youths to use arms and ammunition. Every batch would have around 200 youths of all ages," said an official.
"He has great oratory skills through which he influenced people. He is a sort of motivational speaker who encouraged his disciples to join jehad," said a police officer.
Due to his ability to motivate people, he was greatly sought after terrorist groups.
Tunda told interrogators that to carry out a terror attack, "you need a man, motivation and a bomb. Tunda provided all this and that is why he was so popular among all terror outfits," said the officer.
"His job was to recruit, motivate and provide explosives to youths," the officer added.
Born in a poor family in 1943 at Chatta Lal Miya in central Delhi's Daryaganj, Tunda was running these madrasas for 18 years and training youths for 28 years.
Investigators are trying to find out Tunda's activities since 1998, when he came to India the last time.
Police is also trying to find out why was he was coming to India this time and how Pakistan-based terrorist outfits and Inter-Services Intelligence (ISI) carry out and control terror modules in India.
"His bomb-making skills and motivational power was the reason that led Tunda to meet Hafiz Saeed, Maulana Masood Azhar, Dawood Ibrahim and the Bhatkal brothers of Indian Mujahideen in Pakistan," said the officer.
He also recruited young poor children from Pakistan and Indian youths who fled to Pakistan to join terrorist groups like Indian Mujahideen.
Due to his preachings at various madrasas in Pakistan, he had a huge well knit network of terrorists under him in Pakistan, Bangladesh and Myanmar.
Through this network, he pushed explosives and terrorists into India through the Bangladesh border since 1998.
He was directly in touch with Hafiz Saeed and Zakiur Rehman Lakhvi, and would get funds from the ISI and jehadi individuals.
He met Hafiz Saeed in 1991 and returned to India and carried out several blasts. He last met Hafiz Saeed in Pakistan just 10 days before he was nabbed.
On hearing of Tunda's works, underworld don Dawood Ibrahim met him in 2000 in Karachi for the first time.
"Thereafter, he has met Dawood for around six to seven times. He was handling Dawood's fake India currency notes network," said the officer.