BRASïLIA (AFP) - Brazil's President Dilma Rousseff formally launched her re-election bid on Saturday, seeking a second four-year term in October polls amid discontent over World Cup costs.

Her leftist Workers Party approved her candidacy in a voice vote of 800 members meeting at a convention in Brasilia, with the popular former president Luiz Inacio Lula da Silva on hand.

"It is time to move forward, it is time to make more changes," Ms Rousseff told her party.

Ms Rousseff leads her rivals ahead of the October presidential election, with 39 per cent of voters backing her candidacy, a survey by pollsters CNI Ibope showed on Thursday.

Social Democratic rival Aecio Neves was second with 21 per cent, followed by socialist former governor Eduardo Campos with 10 per cent.

Ms Rousseff's candidacy was made official nine days into the World Cup.

Brazilians held massive demonstrations during last year's Confederations Cup, a warm-up to the main football event, to protest the World Cup's US$11 billion (S$13.7 billion) bill and demand better public services.

The protest movement has lost some steam, drawing smaller crowds during the World Cup but with sporadic clashes between riot police and masked radical protesters.