The Justice Department has threatened to use force to keep demonstrators off Pennsylvania Avenue during the massive anti-war march in Washington on November 15.

Citing reports that militants plan violent confrontations, Deputy Attorney General Richard G. Kleindienst, a Justice Department spokesman, said that anything larger than a "small symbolic procession down the avenue" would be "an illegal act and we would attempt to stop it with minimum of force necessary."

Marines and soldiers numbering over 28,000 are available in the area if they are needed, he added.

Late yesterday Kleindienst offered NMC a permit for the march which would allow the 300,000 expected demonstrators to proceed down Constitution Avenue passing the south area of the White House on route to the Washington Monument for a mass rally. The NMC leaders rejected the offer.

Ted Johnson, an NMC spokesman, said the proposal did not meet NMC's demand that the march pass in front of the White House on the north side, which offers the closest access to the White House.

"We will not compromise on this political symbol," said Christine Stevens, another NMC spokesman.

Kleindienst did not explain why government officials consider Constitution Avenue-which roughly parallels Pennsylvania one block to the south-is more likely to minimize the danger of violence by militants.

The Justice Department had rejected a compromise proposal by NMC earlier yesterday. Under this proposal NMC had dropped its demand to use Pennsylvania Avenue. They had rerouted the march so that marchers would move down Constitution Avenue, circle around the north front of the White House, and gather behind it on the Mall.

John Dean, who represents the government in its negotiations with NMC, saidthat Weathermen, a faction of SDS, may try "to disrupt traffic, to cause vandalism and to close down the downtown business area." He added that other militant groups were planning violent confrontations with federal authorities but refused to identify them.

Kleindienst, Dean's superior, said that some of these militants would come from Harvard. M. I. T., and the University of Wisconsin. "If they engaged in violence there, we must assume that they may engage in violence here," he said.

"That's absurd," said Ted Johnson, an NMC spokesman. "We haven't heard of any plans for violence and we have better contacts on campuses than the FBI." He accused the department of waging a "scare campaign" to discourage protestors from coming to Washington.

Johnson added that NMC will have over 5,000 trained marshals to maintain order during the march.

Miss Stevens, an NAC spokesman, said that except for the major march the NMC has permits for all its planned events including the March Against Death on Nov. 13-14.

"It's hard to imagine the government thinking that a handful of radicals could determine the actions of hundreds of thousands," Miss Stevens added.

Meanwhile at Harvard, the Vietnam Moratorium group here has sent a letter signed by seven professors to all Faculty members. The letter calls for their united support against the war and reminds them of the Moratorium days November 13 and 14.

The letter urges them either to cancel classes, reschedule them, or alter the format of regular sessions on these days. It also asks for volunteers to speak against the war at high schools and community gatherings.