The North Cambridge Vietnam Committee last night completed plans for a demonstration to be held today against the second largest defense contractor in Cambridge, Bolt, Beranek and Newman. Inc. (BBN). Demonstrators will gather at 4 p.m. opposite the Fresh Fond shopping center and march to the firm's headquarters at 50 Moulton St.

Demonstrators will urge BBN to accept no further Defense Department contracts, Sara Bershtel, a teaching fellow in Comparative Literature and a spokesman for the committee, said yesterday.

BBN currently has contracts with the Department of Defense worth $8.6 million, with total sales of $20-22 million expected this year.

BBN is responsible for development, management and maintenance of the ARPA (Advanced Research Projects Agency) computer network. The ARPA network facilitates a nationwide military computer network which, according to the North Cambridge Vietnam Committee, makes automated warfare possible.

One of ARPA's major concerns has been Project Blue Nile, the defense program of weather modification as a military weapon. It is intended to put storm clouds, turbulences and tropical cyclones to useful tactical purposes.

The ARPA network is also being used as an experimental model for a global military command and control system.

Besides ARPA. BBN is also involved in acoustics research for the Navy, a North Cambridge Vietnam Committee spokesman said yesterday. The company helped develop the MK-48 torpedo, whose automatic sonar system was later used in the mines dropped into North Vietnamese harbors last spring, he added.