President Carter had hardly left town yesterday before battle lines began forming over who is going to get how much from his new urban program.

Carter called Monday for an $8.3 billion aid package that would help urban areas generally but would focus on troubled central cities.

Shortly after he departed yesterday on a seven-day, four-nation trip to South America and Africa, various city, staete and country interest groups held meetings to assess the urban aid proposals and determine how they could get the most from them.

Bernard J. Hillenbrand, executive director of the National Association of Counties (NACO), compared the Carter plan with President Nixon's efforts to impound funds that Congress had approved for various domestic programs.

"This is Carter's first impoundment," he charged. "It is a massive response to the problems of the core city at the expense of the counties."

Hillenbrand said NACO has "no difficulties with Carter's proposed new initiatives to help cities, but we are deeply concerned when he says 160 already existing programs have been changed. We want to know what programs and whether they're going to take money away from us. I can't conceive of how Congress would stand for it."

Hillenbrand complained that during White House briefings on the new program and in a report by a task force headed by Housing and Urban Development Secretary Patricia Roberts Harris "the word county did not appear one single time. It was city, city, city."

Meanwhile, one mayor, who had reacted negatively on Monday to the Carter's program's funding levels, reversed himself yesterday.

"I'm somewhat disvowing my initial press release," said Syracuse Mayor Lee Alexander, president of the U.S. Conference of Mayors.

On MOnday Alexander, speaking for the conference, had said Carter's proposals "do not give enough attention to the critical national problems of unemployment, housing and transportation, which are concerntrated in cities."

Yesterday he said, "I'm very enthusiastic about the money." Noting that of the $8.3 billion package, $6.1 billion would be in new spending and tax incentives, Alexander said, "That's better than half of what we recommended." The conference had proposed an $11.3 billion program.

Alexander explained that the conference had prepared the press release before knowing the total figure in the Carter program. A conference spokesman said later, "We're throwing away all our old recommendations. We're flying the white flag. We're giving up."

The National League of Cities, representing mayors and council members of 780 cities, held planning sessions yesterday to determine how fund-distribution formulas would work under the new program. Several mayors stressed that they do not object to Carter's plan to direct some of the new money to neighborhood self-help projects.

While the Conference of Mayors had objected to Carter's proposals for neighborhood and for federal incentives to states that try to help cities, the league reacted positively.

However, Cathy Reynolds, a Denver city council member, said of Colorado's state legislature: "We have some of the finest cowboys in the world there, but they think cities are places to keep pornography in."

At the National Association of Neighborhoods, Executive Director Milton Kotler commended Carter's "recognition of neighborhoods as a major category in the urban policy."

Mayor Richard G. Hatcher of Gary, Ind., president of the NAtional Conference of Democratic Mayors, said he thinks Carter will be helped politically by the urban program.

A recent Louis Harris survey showed that Carter's job rating among big-city residents was 54 to 40 percent negative, a sharp turnaround from 59 to 37 percent positive rating he had received in big cities in July.

"I think the urban program will reverse the president's lower rating in the cities," Hatcher said. "After all, that's where he won big in 1976." The Harris survey noted that Carter won by a 58 to 40 percent majority in the nation's largest cities.