Partial results, after over 67 percent of the ballots were counted, showed President Luiz Inacio Lula da Silva garnering almost 48.8 percent with a lead of nearly eight points over his closest rival.
An Ibope exit poll projected Lula would get 50 percent against 38 percent for former Sao Paulo Governor Geraldo Alckmin.
Lula needs to get over 50 percent to avert going to an October 29 runoff against Alckmin.
Mr Alckmin, 53, was upbeat as he voted earlier in the day, pointing out "opinion polls show there will be a second round."
But Lula, 60, a former strike leader who has distanced himself from his radical past, was unfazed.
"I'm certain we will win today's election," he said after casting his ballot in the industrial town of Sao Bernardo do Campo, the stronghold of his leftist Workers Party (PT) just outside Sao Paulo.
A onetime shoeshine boy with little formal education, Lula enjoys widespread support among impoverished Brazilians, who make up about a quarter of the 184 million population of South America's largest nation.
While dented, his popularity has survived a series of scandals that dogged his government and the ruling PT, thanks in part to an anti-poverty program under which 11 million struggling families get up to 45 dollars a month in government subsidies.
And the financial and business communities no longer perceive Lula as a dangerous firebrand leftist.
Since Lula took office in 2003, the government has maintained orthodox economic policies and restored public finance, while increasing the minimum wage.
The elections are being held against the backdrop of a healthy economy, though Alckmin claims growth, projected at three percent this year, is far short of its potential.
The elections coincided with a three-day period of mourning for the 155 passengers and crew of a Brazilian airliner feared to have perished when the Boeing 737 crashed in a remote, densely forested area.