Former state senator Kasim Reed did have the slight lead with 99 percent of precincts reporting, but Norwood told supporters that she was not conceding and was open to a recount. Both had 50 percent of the vote.
Under Georgia law, the runner-up can petition for a recount when the margin of victory is less than 1 percent of the total vote. Voters cast 84,076 ballots, so the winning margin would be less than 1 percent.
Reed, who is a black, and Norwood, who is a white, have waged a hard-fought battle across the city and over the airwaves in the month leading up to the runoff. Both tried to gain a critical mass of racial crossover votes, with victory likely hinging on black-versus-white turnout.
Reed was claiming victory at his downtown campaign headquarters early Wednesday.
"Guess who's going to be the 59th mayor of Atlanta?" he said to supporters.
Norwood was not yet ready to make any declarations about the outcome.
"Tomorrow we will see where it all ends up," she said. "It is closer than any of us thought and it has been a harder campaign than any of us thought. We have not gotten enough good information from the county to know that all the votes have been counted."
The city's changing racial demographics could have played a role in the outcome. In 2000, Atlanta was 33 percent white and 61 percent black. In 2007, the numbers were 38 percent white and 57 percent black, according to the U.S. Census.
The winner will inherit a list of challenges after taking office on Jan. 4.
They include the city's sagging finances, easing citizens' fears about crime, fostering a working relationship with state lawmakers and returning the city to its reputation as the jewel and economic engine of the South and The City Too Busy to Hate.
Reed, 40, who resigned from the Senate to run, steadily gained momentum in the runoff, with a blitz of endorsements that have kept his name in the local media. Reed raised more money than Norwood during the runoff, but Norwood, 57, had more cash on hand entering the final stretch of the campaign. The latest finance reports showed Norwood spent about $566,000 in the runoff compared to Reed's $790,000.
In November, voters were barraged with ads and phone messages and the race dominated local media. Jessica Gardom, said the frequent Robocalls about the campaign were a turnoff.
"I think I voted for the people who called me least," said Gardom, who voted for Norwood who owns an automated telephone calling business.
Norwood, a former radio executive and a Junior League veteran, ran a grassroots campaign as "an outsider," despite having served in an at-large post on the city council since 2001. Tom Austin, an attorney who voted for Norwood, said he felt she represented a change in Atlanta politics.
"I believe that Mr. Reed is just more of the same that we've had for the past several administrations," Austin said.
In the campaign for the Nov. 3 general election, Norwood was seen as the front runner and some speculated that she might win outright. Instead, the crowded field of candidates including many blacks who split the city's African-American vote forced a runoff.
Reed has modeled himself in the tradition of iconic Atlanta mayors Maynard Jackson elected the city's first black mayor in 1973 and former U.N. Ambassador Andrew Young, a lieutenant of the Rev. Martin Luther King Jr. He was seen by many as the heir apparent to the Jackson legacy, which also includes current Mayor Shirley Franklin.
Odie Donald considered voting for both candidates, but ultimately went with Reed, whom he also voted for in the general election. The 31-year-old Atlanta native said he admired Norwood's commitment to the city, but chose the former lawmaker because he felt Reed would get things done. Donald, who is black, said race was not a deciding factor.
"I can't necessarily say that weighed heavily into my decision," Donald said. "It's like an added bonus."