Her vice president, Michel Temer, has been interim president since mid-May, when Rousseff was suspended after Congress decided it would continue the impeachment process that began in the lower house.
Rousseff's testimony was her last chance of defense in a almost nine-month impeachment process that has plunged Latin America's largest economy into political turmoil and deepened a recession.
Rousseff is accused of issuing, in 2015, decrees for supplementary credit without the approval of congress and of using funds from state-owned banks in federal Treasury budgetary programs, which has been referred to as "fiscal pedaling" (budgetary manipulation).
And it's because of my absolutely clear conscience about what I did, in the exercise of the presidency, that I have come personally before those who will judge me. "I did not commit a crime", Rousseff told senators who listened intently, in contrast to the chamber's usual raucousness.
Brazil's suspended president laid out the case in her defense in a 30-minute speech before the Senate Monday morning ahead of taking questions from senators.
"I'm here to look in your eyes and say with the serenity of someone who has nothing to hide that I haven't committed any crimes", Rousseff told her accusers.
Presenting her defense at an impeachment trial in the Senate, Brazil's first female president said the economic elite had sought to destabilize her government since she narrowly won re-election to a second four-year term in 2014.
Outside Congress, a huge wall was put up to separate Rousseff supporters and pro-impeachment activists.
Yet many top leaders, including Silva, have acknowledged that Rousseff's chances of surviving the Senate's final vote are slim.
A man carries a banner with the name of Brazil's suspended President Dilma Rousseff at a camp in Brasilia, Brazil, Sunday, Aug. 28, 2016. "I became bitter", Ms Rousseff noted, her voice cracking sometimes as she spoke. While she isn't accused directly of profiting, Rousseff was the chairwoman of the state oil company during numerous years of the alleged corruption.
For Rousseff to be removed, at least 54 of the 81 senators must vote in favor.
The senators need to reach 54 of votes to impeach her.
The beleaguered president was suspended from office in mid-May, when the Senate voted to hold the trial that is now in its final stage. The result is expected to mirror the last vote when the Senate approved the impeachment process by 59 votes to 21. "But among my defects is not disloyalty or cowardice", she said.
She took to the stand for around half an hour, before facing questioning from allies and opponents. If the Senate convicts Rousseff on Tuesday or Wednesday as expected, Temer, 75, will be sworn in to serve out the rest of her term through 2018.
Rousseff is accused of carrying out an allegedly illegal tax maneuver but she insists the claims against her are nothing more than a political conspiracy to oust her from office and roll back more than a decade of social policies that helped the country's poor.
They ignored warnings to stay silent from Supreme Court Chief Justice Ricardo Lewandowski, who was presiding, bursting into applause and later chanting: "Dilma, warrior of the Brazilian people!"
Associated Press writer Mauricio Savarese reported this story in Brasilia and AP writer Peter Prengaman reported from Rio de Janeiro.