"Vote against impeachment, vote for democracy... Do not accept a coup," the 68-year-old leftist leader said as she defended herself before senators who are widely expected to remove her from office.
Rousseff - Brazil's first female president - was testifying for the first time at her trial, hours before senators were to start voting on her fate.
All indications point to her being removed from office, ending 13 years of rule by the leftist Workers' Party in Latin America's biggest country.
"I've come to look your excellencies in the eye and to say that I did not commit a crime," Rousseff, 68, said in a calm, firm voice from the Senate chamber podium.
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"I did not commit the crimes for which I have been accused unjustly and arbitrarily."
Rousseff is accused of having taken illegal state loans to patch budget holes.
But momentum to push her out of office is also fueled by deep anger at Brazil's historic recession, political paralysis and a vast corruption scandal centered on state oil giant Petrobras.
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