Police fired tear gas and arrested more than 3,000 passively resisting protestors Friday in an attempt to break up the largest antinuclear demonstration ever staged in the United States. More than 123,000 demonstrators confronted police on the construction site of a 1,000-megawatt nuclear power plant scheduled to provide power to most of southern New Hampshire. Organizers of the huge demonstration said, the protest was continuing despite the police actions. More demonstrators were arriving to keep up the pressure on state authorities to cancel the project. The demonstrator had charged that the project was unsafe in the densely populated area, would create thermal pollution in the bay, and had no acceptable means for disposing of its radioactive wasters. The demonstrations would go on until the jails and the courts were so overloaded that the state judicial system would collapse.