By Steve Kingstone
BBC News, Sao Paulo
Brazilian President Luiz Inacio Lula da Silva has given his first public reply to allegations that his party offered bribes for parliamentary support.
President Lula promised a thorough investigation
The claims against President Lula's governing Workers' Party were made on Monday by the leader of another party.
Speaking in Brasilia on Tuesday night, Lula said the votes of lawmakers should never be up for sale.
He said he personally would ensure that investigations reached their logical conclusion.
By coincidence, the president was addressing an international forum to discuss ways of combating corruption.
On paper this was an uncomfortable encounter in which a president facing bribery claims addressed international delegates who have come to Brazil to discuss corruption.
No outright denial
Initially looking nervous and drawn, Lula nonetheless offered a spirited performance.
"I swore by the constitution to be the main guardian of Brazil's institutions," he said, "and the parliament cannot ever be for sale."
The president promised a thorough investigation, adding there would be no cover-up.
"If needs be, we are prepared to cut off our own flesh," he said - an apparent reference to alleged wrongdoing within his own Workers' Party.
Tellingly, there was no outright denial of the allegation that the party treasurer made payments to lawmakers from other parties but neither has there yet been any hard evidence to support the claim.
The treasurer, Delubio Soares, has been kept away from the media since the story broke.
He is expected to give his version of events on Wednesday.