The defendants are accused of involvement in a scheme that used public funds to pay coalition partners to support the government's agenda.

Among those accused are former leading members of Lula's Workers' Party (PT). All reject the charges.

At the time, Lula denied knowledge of the scheme and said he felt betrayed.

The case, already dubbed the "trial of the century" by the Brazilian media, involves 38 former members of the PT and other political parties, government officials, business people and bankers.

They face a range of charges including money-laundering, corruption, and accepting bribes.

It is being heard by the 11 Supreme Court justices sitting in Brasilia.

Ahead of the proceedings, the Attorney General Roberto Gurgel sent the judges a note describing the case as "the most daring and outrageous corruption scheme and embezzlement of public funds ever seen in Brazil".

The scandal was known as the "mensalao" or "big monthly allowance".

Members of the PT are alleged to have paid political allies some $10,000 (£6,400) each every month to ensure they voted through the government's agenda in Congress.

Prosecutors allege that the money was diverted from the advertising budgets of state-owned companies.

The scandal broke in 2005 during the first term of President Luiz Inacio Lula da Silva, leading to the downfall of several senior members of the government.

These included Jose Dirceu, who was Lula's chief of staff. He was replaced by Dilma Rousseff, who went on to be elected as president in 2010. She is not implicated in the case.

Mr Dirceu's lawyer has said there was no "so-called vote-buying".

"There is no proof of any use of public money. Dozens of witnesses categorically say that Dirceu had no knowledge of the loans and (money) transfers," Jose Luiz Oliveira Lima told the newspaper O Globo.

The trial, which is expected to last a month, takes place just a few weeks before important municipal elections.

Analysts say it could tarnish the reputation of Lula, who served as president from 2003 to 2010, and who remains hugely popular in Brazil.

But while the PT and its allies are in the dock, the alleged corrupt practices cast a negative light on the whole political establishment, correspondents say.