Antiwar demonstrators may not march past the United Nations complex on Saturday, or anywhere else in Manhattan, a federal judge ruled yesterday. Agreeing with the city that a large, moving rally of 100,000 people or more raised serious security risks, the judge said the organizers would have to settle for a stationary rally five blocks north of the complex. In refusing to grant a parade permit, the city did not violate the demonstrators' First Amendment rights, the judge, Barbara S. Jones of Federal District Court in Manhattan, wrote in her opinion rejecting their request for a preliminary injunction. She said that their free-speech rights were adequately protected by the city's counteroffer of a rally for 10,000 people at Dag Hammarskjold Plaza, at 47th Street, with overflow space as far north on First Avenue as needed.