The nation's big city mayors yesterday joined the chorus of critics attacking President Reagan's budget proposals, charging that their relatively modest share of federal expenditures has been targeted for two-thirds of proposed spending cuts.

The Reagan budget "is a disaster for the cities," the U.S. Conference of Mayors declared in a 76-page critique of the budget made public by the conference president, Mayor Richard G. Hatcher of Gary, Ind.

Specifically, Hatcher complained that while state and local grants account for 14 percent of the total federal budget, those categories of spending are earmarked for 66 percent of cuts proposed by the administration.

"If this budget goes through, we will keep 1 million people from jobs and job training next year," he said. "If this budget goes through, we will kill the dream of an affordable, decent home for more than 35,000 families next year. We will take food off the table of 400,000 households involving many people who are just above the poverty line."

In place of these programs, Hatcher said, "we will get more weapons for the military, [and] we will also get longer unemployment lines and welfare rolls, with the bill going to the average taxpayer. The administration will put money in our pocket with one hand and take it out with the other."

Hatcher said the mayors support the concept of consolidating several categorical grant programs into single block grants to states and cities, which Reagan has proposed. But he said the administration's proposed block grants are really "deep budget cuts dressed up to look like block grants."

Moreover, he warned, having the block grants go first to state governments for later dispersal to local governments could "set local, county and state governments at each other's throats."

There are, in fact, differences among officials at various levels of government about Reagan's proposals. The conference of mayors generally represents the interests of the nation's largest and most troubled urban centers, and yesterday's statement was among the most critical in response to the proposed budget cuts.

In contrast, the National Governors Association yesterday issued a relatively bland analysis of the budget proposals that took no position on the cuts. And the National League of Cities, which is heavily influenced by the interests of officials from small and medium-sized cities, earlier said it supports Reagan's attempts to cut the budget and is willing to do its "fair share."

The mayors were joined in their criticism of the administration yesterday by the National Association of Latino Elected and Appointed Officials, which charged that the interests and needs of the Hispanic community are geing ignored in formulation of economic policy and in making appointments to government positions.

At his news conference, Hatcher said there is "a kind of wonderful illogic" to some of the proposed cuts. He cited specifically the proposed elimination of the Economic Development Administration, which he said should be supported by an administration that favors encouragement of private investment. s

The Reagan budget-cutting, he said, "amounts to almost a religious crusade to get government out of people's lives almost totally" that is being waged by "fervent true believers" in that philosophy.