Maharashtra's Anti-Terrorist Squad on Monday arrested Asif Khan Bashir Khan alias Junaid, suspected to be the mastermind of the July 11 train blasts.
He was picked up from Belgaum district in northern Karnataka.
An ATS officer said: "Junaid was the prime person in the entire conspiracy."
With his arrest, the number of suspects arrested in connection with the blasts has gone up to 16.
Junaid, suspected to be an activist of the banned Students Islamic Movement of India, has been brought to Mumbai, the official said, without elaborating on his exact role in the terror attacks on Mumbai's lifeline.
Police sources said Junaid is from Jalgaon in northern Maharashtra and is believed to be one of the three absconding Indians who had allegedly planted the bombs along with Pakistanis.
Junaid is also wanted in a criminal case in Jalgaon concerning alleged subversive activities of Lashkar-e-Tayiba.
Seven bombs ripped through Mumbai's suburban trains during peak hour, killing nearly 200 people.
The police have said the attack was planned by Pakistan's ISI and executed by the Lashkar and SIMI.
