The election pits incumbent Fauzi Bowo against the popular current mayor of Solo city in Central Java province, Joko Widodo.

Widodo topped the vote in July's first round -- winning 43 percent of votes, with Bowo taking 34 percent -- but failed to reach the 50 percent required for victory.

Widodo is backed by the country's second largest political party, the Democratic Party of Struggle, and the Great Indonesia Movement Party led by Prabowo Subianto, a former special forces commander.

Bowo ran with the support of President Susilo Bambang Yudhoyono's Democratic Party.

Analysts have predicted a close race. Burhanuddin Muhtadi, a researcher at local polling agency Indonesia Survey Institute, said wealthier Jakartans, who make up a fifth of eligible voters, are expected to pick Widodo.

"They are the ones who can afford to own cars so they know how bad the traffic congestion is. They feel that Fauzi Bowo has failed to solve it during his term," he told AFP.

Voting began at 0000 GMT and will continue until 0600 GMT.

There are seven million registered voters. Ballots will be counted by hand across 15,000 polling stations after voting closes, but the official results will only be announced on September 29.