The nation's mayors said yesterday that, because of the forthcoming Reagan budget cuts, more than two thirds of 100 major cities just surveyed are already beginning to lay off workers and over four-fifths predict a negative impact on the poor.

"The cuts are much deeper and much worse than anyone thought initially," said Mayor Richard Hatcher of Gary, president of the U.S. Conference of Mayors. Hatcher said two-thirds of the president's proposed $48 billion in spending cuts next year would come at the expense of cities, and not just large ones but many small ones in the South, West and elsewhere.

Mayor William Hanna of Rockville, Md., said the cities are the "critical mass" for productive capacity, that yield most of the nation's wealth. If the governemnt fails to put back some of the money that it taxes away and that suburban-dwelling commuters take out, Hanna said, then the nation's productive capacity will decay and President Reagan will have undermined his goal of reviving the economy.

The 100-city survey, presented at a news conference, showed:

92 percent of those responding predicted ill effects on their educational systems, and many said they would have to lay off police, fireman and sanitation workers.

The District of Columbia anticipated laying off 1,086 people in public service jobs funded through the Comprehensive Employment and Training Act, Baltimore 3,000, Richmond 170, Rockville 22, New York 11,500, Los Angeles 4,500 and Dade County (Fla.) 6,500.

Two-thirds said they'd have to increase public transit fares, and most said they'd also have to reduce transit services. Three quarters said new housing for the poor and maintenance of existing public housing would be sharply hurt.

57 percent said their economic development plans or urban rehabilitation would be crippled or severely hurt. New Orleans said it could lose as many as 15,000 jobs over the next three years as a result, Louisville feared loss of a downtown development project intended to create 500 jobs. Dayton, Ohio, whose mayor, James McGee, told the news conference that civil disorder is possible this summer because of the cuts, said the freezing of funds for a north-south Dayton thoroughfare had shoved "down the tube" commitments from businesses to stay in the city.

In education, Richmond said it would have to cut services for handicapped students and follow-up programs for Head Start children; Kansas City would have to slash staffs 20 percent; Baltimore said it would have to cut back clinical services, pre-kindergarten classes, intensive reading, counseling and programs for "exceptional children." Boston, which has already run out of school money, will lose $4 million in federal funds next year.

Many said enrollments in urban colleges serving relatively low-income people probably would drop substantially on account of college aid cuts. Fresno predicted a 40 percent drop at the University of Fresno; Nashville said that 40 percent of the nation's black doctors are trained at Meharry Medical College there and student-aid and loan losses would have a severe impact.