By Tom Gibb
BBC News, Sao Paulo
Some 12,000 to 15,000 left-wing protesters have marched in Brazil's capital, Brasilia, to demand the resignation of the president.
Protesters accuse the government of betraying election promises
Brazil's embattled President Luiz Inacio Lula da Silva is fighting to survive a corruption scandal.
His governing Workers' Party has issued an apology to the nation.
The party is accused of paying for the support of allied parties in the Brazilian Congress and using illegal, undeclared election funds.
The march by left-wing parties which have broken away from the government was slightly larger than a rally in support of President Lula da Silva earlier in the week.
The protesters called on Lula to step down or be impeached by Congress.
They accused him of betraying election promises to promote change and redress Brazil's huge wealth gap.
So far, however, the protests, both for and against Lula, have been too small to really affect the course of the corruption scandal.
Some opposition lawmakers have said they will only support impeaching Lula if public opinion demands it.
Polls have shown the president's support tumbling, but at the same time the majority of Brazilians do not want to see all the political and economic instability that an impeachment would entail.
Meanwhile, the Workers' Party has issued a public apology to the nation, saying that its members were not aware of the practices being followed by some of the leadership.
While the statement supports Lula, it also contains implicit criticism of the way the party has been run, calling for greater accountability.
Many in the party would like to see the government change its highly conservative economic policy, which has closely followed IMF-style austerity measures.