President Luiz Inácio Lula da Silva spoke with nostalgia to more than 10,000 supporters at the Vila Euclides Stadium outside Sao Paulo — a city where he rose to political fame 47 years ago as a metalworkers union leader during the country's 1964-1985 military dictatorship.
The 80-year-old, wearing a hat after a skin cancer diagnosis, insisted Brazil needs more of his policies.
Senator Flávio Bolsonaro, one of the sons of former president Jair Bolsonaro, gathered with thousands of voters on Copacabana Beach in Rio de Janeiro.
His father, under house arrest following his conviction for an attempted coup, did not attend the rally.
Lula echoed his own earlier comments denouncing foreign interference in Brazil's politics.
His allies have accused the US government of seeking to boost Flávio Bolsonaro's bid with measures such as raising tariffs on Brazilian exports and revoking the visa of Brazil's ambassador to Washington.
Speaking about Brazil's vast reserves of rare minerals, Lula said: "No one (abroad) will tap our minerals. We are going to tap them for the benefit of the Brazilian people".
Bolsonaro argued that he is: "the only one who can sit with the United States, with China, with Israel, with the Middle East, with India and with Argentina to make the best and biggest trade deals for our nation".
His party's convention was attended by Argentina's President Javier Milei.
Israeli Prime Minister Benjamin Netanyahu and several members of the Trump administration have expressed support for him.
Sunday was the first campaign day for the 13 candidates in the running, but many voters are expected to pay more attention once campaign ads start airing on TV and radio at the end of the month.
"It looks like we are starting slow this time," said 68-year-old Flávio Alves, a Lula supporter who lives on the same street where the rally was underway.
"We all expect a narrow election, but now Lula is in government, things are different."
Four years ago, Lula beat incumbent Jair Bolsonaro with 50.9 per cent of the vote.
Another presidential candidate, Ronaldo Caiado, a former state governor and an agribusiness leader, toured the surroundings of Brazil's capital Brasilia and reinforced his anti-Lula credentials.
Another Lula opponent, Romeu Zema, met voters and attended a Catholic ceremony in the countryside of Minas Gerais, considered a bellwether state for the elections — a state he had governed until recently.
Lula, who is seeking his fourth non-consecutive term, and Bolsonaro each reached out to women voters, who account for nearly 53 per cent of Brazil's 158 million eligible voters, according to the country's electoral court.
"Men need to learn from childhood that they are no better than women. We need to be equal," said Lula, as first lady Rosângela da Silva stood by his side, nodding.
She was one of the event's main speakers in Sao Bernardo do Campo, where Lula lived a large part of his life.
Lula said he will launch his platform within 10 days, with more details to come about his policies to benefit women, including education initiatives and policies to prevent and punish violence against women.
Senator Bolsonaro, with his mother and wife by his side, told supporters he would work to fight violence against women.
Flávio Bolsonaro seeks to distinguish himself from his father. In his speech, the younger Bolsonaro vowed to fight crime and to remove what he described as left-wing "ideology" from Brazilian schools.
Every student "has the right to dream of being Elon Musk someday," he said.
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