KARACHI, Pakistan, April 3 (AP)—Informed sources said here today that Sheik Mujibur Rahman was being held in custody in West Pakistan.

He reportedly was flown out of East Pakistan the day of his arrest a week ago in Dacca. Resistance leaders have said he is free, but his voice has not been heard on broadcasts attributed to them.

Sheik Mujib, who is 51 years old, has been branded a traitor, and President Agha Mohammad Yahya Khan has said he must be punished.

The army is reported to be detaining at least 24 members of the working committee of Sheik Mujib's party, the now banned Awami League, which held 167 of the 313 seats in the National Assembly, whose sessions have twice been postponed.

The assembly was elected last December to draft a constitution that would end military rule in Pakistan. The Awami League won a commanding majority with its campaign for regional autonomy.

The first session was to have been held on March 3. The postponement touched off strikes and other protest action in East Pakistan, The President rescheduled the assembly opening for March 25, but again it was put off and Sheik Mujib re fused to attend until military rule was ended.

His Awami League gradually took control of East Pakistani affairs. When the army moved March 25 to reassert the authority of the central military government based in West Pakistan, Sheik Mujib announced the formation of an independent Bengal.