A controversial regulation reserving 75 percent of federal urban block grants to any community for low and moderate-income neighborhoods was made final yesterday, but in softened form.

Instead of making the 75 percent an absolute requirement, Secretary of Housing and Urban Development Patricia Roberts Harris said cities, towns and urban counties will be allowed to fall below the 75 percent figure if they can demonstrate to HUD that their proposed use of funds will still benefit "principally" low-and moderate-income persons and areas.

Aides said this means spending on low-and moderate-income areas could fall to half the funds, depending on the local situation.

Although the regulations are somewhat softer than those Harris first proposed last October, they are still a substantially stronger requirement than ever that the $3.6 billion in community block grants be focused mainly on the poor, urban experts said.

The regulatons spell out for the first time that the intent of the program is primarily to help low- and moderate income persons and areas (those neighborhoods were income is below the median income for the metropolitan area receiving funds).

They also establish as the general rule that 75 percent of the funds received by a community must be targeted for projects aiding such persons and neighborhoods.

Funds under the program, which began in 1974, are used to improve public facilities like lighting, rehabilitate housing, prepare sites for new housing, plan community development, improve, sewers, create senior citizen service centers and similar projects.

Some spokesman for cities and counties, including the U.S. Conference of Mayors and National League of Cities, complained in Octobr, when the 75 percent figure was first proposed by HUD, that it was too rigid and didn't allow city governments enough flexibility in use of the funds - especially in communities which lack high concentrations of poor persons. The alternation made by Harris in issuing the final regulations was seen as a concession to the complaints from these groups.

Tom Gale, National Urban League housing director, said the new regulations are "weaker" than those proposed last October, but the real test will be on how carefully HUD reviews proposals to go under 75 percent, to see if they are really justified.

Harris said she wanted to make "unmistakably clear that the intent of this multibillion-dollar program is to be responsive to the needs of low-and moderate-income persons, and we will scrupulousy monitor's a community's performance."