Lutfur Rahman, a college teacher and close associate of Bangla Bhai, chief of the Jagrata Muslim Janata Bangladesh (JMJB), was detained in the northern Bagmara area on Monday.
Police said on Wednesday Rahman was the principal adviser to JMJB, which the government banned in February along with Jamaat-ul-Mujahidin for criminal activities.
The government believed Jamaat-ul-Mujahideen was mainly responsible for the wave of suicide bombings that have killed at least 30 people and wounded 150 since 17 August.
On Tuesday, police detained three Jamaat-ul-Mujahidin members and seized 10kg (22 pounds) of explosives from them at Narayanganj, 18km (12 miles) from the capital, Dhaka.
Police said they had detained nearly 800 suspects, mostly Jamaat-ul-mujahidin activists, over the past four months. But Jamaat-ul-Mujahidin leader Shaikh Abdur Rahman and Bangla Bhai are both still at large.