ISLAMABAD, Pakistan--The Pakistani government imprisoned 48 opposition leaders yesterday to forestall a demonstration that the Pakistani National Alliance, the opposition coalition, had planned for tomorrow.

National Alliance leaders said yesterday that demonstrators from all major cities will converge on Islamabad Tuesday to call for the resignation of President Zulfikar Ali Bhutto and his government.

Nawabzada Khan, the leader of the National Alliance, who was among those arrested yesterday, has tried to rally support among army generals, and the National Alliance has asked the army not to follow Bhutto's orders during the upcoming demonstration.

Pakistan's three largest cities, Karachi, Lahore and Hyderabad, have been under martial law for three days, and the government has imposed censorship on all news reports.