"I am appealing for there not to be violence," she was quoted as saying by Globo and other major Brazilian news sites while touring the scene of deadly floods near Sao Paulo yesterday.
"I think all people have a right to be on the streets. And no one has a right to be violent. No one," she said.
More than a million people were expected to take to Brazil's streets Sunday to call for Rousseff's ouster over a corruption scandal and severe economic recession.
There have been growing fears of violent clashes between Rousseff supporters and the opposition as pressure grows in Congress to impeach her.
However, Rousseff sought to calm tensions, saying "I think that tomorrow's event should be treated with respect."
Recalling her past as a leftist guerrilla who was tortured under Brazil's military dictatorship, Rousseff said: "We live at a time when people can demonstrate, they can express what they think -- and this is something we should preserve."
