Former U.N. ambassador Andrew Young and state Rep. Sidney Marcus were headed for a runoff tonight with most of the vote counted after a heavy turnout in the race to succeed Mayor Maynard Jackson.

In a see-saw battle with 186 of 188 precincts in the city reporting, Young led the seven-candidate field with 43,067 votes, or 41.1 percent. Marcus had 40,444 votes, or 38.6 percent.

Fulton County Commissioner Reginald Eaves, a former Atlanta public safety commissioner, was a distant third. He made heavy inroads in the city's mostly black precincts and came away with 15 percent of the total vote, enough to deny Young a clear majority.

The four other candidates, Mildred Glover, Andree Kahlmorgan, Warren Shulman and John Thompson, shared about 4 percent of the vote.

A breakdown of the voting patterns showed that Young carried the black precincts with about 60 percent of the vote, but his strategists had said Young needed at least 70 percent to avoid the runoff.

Eaves' vote in the black precincts ranged from 25 to 30 percent.

Young and Eaves, both black, were expected to divide most of the votes in the predominantly black precincts, while Marcus, a white, liberal businessman, was expected to score heavily in the white precincts of the city's north side.

Fulton County Elections Supervisor Tom Malone reported earlier that the turnout was "heavy throughout the city," and he estimated that more than 60 percent of the city's 191,000 registered voters went to the polls under fair skies.

The ballot also featured city council and school board races, but the attention went to the nonpartisan mayor's race.

Most pre-election observers expected that no candidate would get a majority of the vote, and many predicted a runoff along racial lines between Young and Marcus. Young had predicted that he would win a majority. The runoff will be Oct. 27.

Jackson, Atlanta's first black mayor since Reconstruction, has served two four-year terms and is prohibited from running again. He supported Young in the race.

Albuquerque, Anchorage, Springfield, Mass., and Greensboro, N.C., also chose mayors today.

Mayor Theodore Dimauro of Springfield was ensured of a spot on the Nov. 3 general election ballot in his run for a third term in a city that turned a multimillion-dollar deficit into solvency during his tenure.

In Greensboro, John W. Forbis led a three-way rac Atlanta's first black mayor since Reconstruction, has served two four-year terms and is prohibited from running again. He supported Young in the race.

Albuquerque, Anchorage, Springfield, Mass., and Greensboro, N.C., also chose mayors today.

Mayor Theodore Dimauro of Springfield was ensured of a spot on the Nov. 3 general election ballot in his run for a third term in a city that turned a multimillion-dollar deficit into solvency during his tenure.

In Greensboro, John W. Forbis led a three-way race in which Sol. M. Jacobs was eliminated from centention. Forbis and V.M. Nussbaum Jr. will meet again in November.

Albuquerque Mayor David Rusk, son of former secretary of state Dean Rusk, conceded late tonight that he had lost his bid for reelection, and with half the vote was running well behind former television commentator Gordon Sanders. And in Anchorage, eight candidates were in the race to succeed Gorge Sullivan, prohibited by law from holding a third term.

About 66 percent of Atlanta's more than 400,000 residents are black.

Young, 49, rose to prominence during the civil rights movement of the 1960s as a top aide to the late Dr. Martin Luther King Jr. He was elected to Congress three times in the 1970s from Atlanta's 5th District before resigning to take the U.N. post under then-president Jimmy Carter.

Young's clandestine meeting with a representative of the Palestine Liberation Organization caused controversy, and he resigned the U.N. post and returned to Atlanta, where he set up a consulting business.

Young pledged during the campaign to "show Atlanta businessmen how to do business abroad and create jobs." Much of his campaign was financed through fund-raising events in New York, Washington, Los Angeles, Chicago and Boston.

Marcus, 53, built his reputation as a liberal advocate of Atlanta causes during 13 years in the Georgia Legislature. During the campaign, Marcus pledged to use his experience to secure state funds to replace the federal dollars the city expects to lose under President Reagan's budget knife.

In September, there were disclosures that Marcus had been habitually late in paying city property taxes on his home and on property owned by his home-remodeling business. He acknowledged the late payments and said he paid penalties.

Marcus had drawn the financial backing of the city's white business establishment.

Eaves, 47, campaigned on his administrative ability and his record as Atlanta public safety commissioner, a post that earned him the respect of the city's black community for his role in ending what was perceived as police brutality against blacks.