By Tom Gibb BBC correspondent in Sao Paulo

The most important phase of Brazil's presidential election campaign has got under way with the start of free TV air time for the candidates.

Lula is leading the presidential race

The first day started with an hour of publicly funded air time divided between the presidential candidates. This will now happen three times a week.

With a population of more than 170 million, the sheer size of South America's largest country makes television and radio decisive in campaigning.

Brazil goes to the polls at the beginning of October, but already the prospect of an opposition candidate winning has been wreaking havoc on financial markets.

'Lagging behind'

This election will end eight years in power by Fernando Henrique Cardoso, who strengthened Brazil's democratic institutions while sticking closely to the economic policies of the International Monetary Fund.

Gomes is known as a "star" candidate

Under the electoral system, his chosen successor, former Health Minister Jose Serra, has by far the most free TV air time.

But he is lagging way behind in opinion polls, which show most Brazilians want a change.

The left-wing Workers' Party candidate, Luiz Inacio Lula da Silva, or "Lula", is leading the polls.

A former union leader, this is his fourth attempt at becoming president.

In the past, he criticised payment of the international debt, but in this campaign he has put on a suit and tie and has dramatically moderated his platform.

Close behind him is Ciro Gomes, a former governor from the north-east of Brazil.

While much of his rhetoric comes from the left, his supporters include some of Brazil's most conservative politicians, leaving many unsure as to what kind of government he would lead.

However, young and charismatic, with a girlfriend who is a popular soap-opera star who has beat cancer, he has risen quickly and dramatically in the polls.