Incumbent President Dilma Rousseff faced her main rivals Marina Silva of the Brazilian Socialist Party (PSB) and Aecio Neves of the PSDB.
Dilma Rousseff is ahead in the polls for the October 5 election but analysts predict it will go to a second round.
Marina Silva has replaced the former PSB candidate Eduardo Campos, who died in a plane crash earlier this month.
In the much-anticipated debate, President Dilama Rousseff defended her government’s popular social programs but blamed the international economic crisis for the country’s slowing economy.
Marina Silva spoke passionately about the need for widespread political reform and to remove power from the hands of traditional elites.
The latest opinion polls in Brazilian media on Tuesday showed Marina Silva had narrowed Dilma Rousseff’s lead to 34%, down from 38% in early August.
It gave Marina Silva 29% of voter support and Aecio Neves 19%.
The poll, published by O Estado de Sao Paulo newspaper, has a margin of error of plus or minus 3%.
Tuesday’s televised debate also includes candidates Eduardo Jorge (PV), Levy Fidelix (PRTB), Luciana Genro (PSOL) and Pastor Everaldo (PSC).
Dilma Rousseff is seeking re-election with the Workers’ Party (PT).
Marina Silva used to be a member of the PT during the government of Luiz Inacio Lula da Silva, but has since become a stern critic.
A former environment minister, Marina Silva ran for president in 2010 for the tiny Green Party and secured 19 million votes, but was knocked out in the first round.
Marina Silva decided to run for vice-president alongside Eduardo Campos after the Electoral Court in October 2013 refused to register her political movement, Rede Sustentabilidade (The Sustainability Network).
However, Marina Silva was appointed as the PSB’s main candidate after his sudden death.
Eduardo Campos died after a private jet crashed in bad weather in the port city of Santos, Sao Paulo state, while travelling from Rio de Janeiro to the seaside resort of Guaruja.
Brazil’s presidential election will go to a second round later in October if no candidate gets more than 50% of the votes.
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