"Today is a national day of shame, a day on which senators started to rip apart the country's constitution and a day on which they begin to debate punishing an innocent woman, whose only crime was being honest," he said on Thursday.
According to Lula, the senators are not trying to depose Rousseff, but instead cancelling the votes of the 54 million citizens who re-elected her in 2014.
Lula also criticized the administration of interim President Michel Temer, which he sees as trampling the Constitution to grab power, Xinhua reported.
"They found a way to get to power without having to run in an election, via a coup in the Congress. I have nothing personal against Temer. I just wish he knew that it would be dignified if he, as an expert in constitutional law, did not accept to come to power through a coup. He should say he is going to run in 2018, to find out whether he would be elected by the vote of the Brazilian people," he said.
The former president also criticized measures taken by Temer, including privatizations of state-owned assets and the divestment campaign of Petrobras.
Rousseff's impeachment trial started on Thursday with the testimonies of witnesses. This phase is expected to last until next Monday, when defense and accusation will present their statements. The impeachment vote is expected to take place Tuesday.
In order to impeach Rousseff, her opponents need a two-thirds majority in the Senate -- the votes of 54 out of 81 senators.