The Senate trial was opened by Supreme Court president Ricardo Lewandowski half an hour late in the blue-carpeted chamber at 9:30 am.
The proceedings in the capital Brasilia were considered almost sure to result in Rousseff, 68, being found guilty of cooking the budget books to mask the depth of economic problems during her 2014 reelection campaign.
If she is removed from office, her former vice president turned rival Michel Temer will be sworn in to serve until 2018, shifting Brazil to the right after 13 years of leftist rule under Rousseff's Workers' Party.
"We will fight to reinforce democracy in our country with the same force that I fought against the military dictatorship," she told supporters yesterday in Brasilia.
The trial will climax Monday when the president, who was suspended from office in May, addresses the Senate herself for the first time. A vote is then expected within 48 hours, with a two thirds majority of the 81 senators required to bring Rousseff down.
Senator Raimundo Lira, a Temer ally and strong backer of impeachment, told AFP that senators "have already made up their minds and I don't think there will be any change at the vote.