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8 year oldShe is accused of illegally manipulating the budget to hide a growing deficit.
Ms Rousseff said her conscience was "absolutely clean" and that she had not committed any crime.
Senators are due to vote later this week on whether to remove her from office for good or whether to reinstate her.
Ms Rousseff began her defence by reminding Senators that she had been re-elected by more than 54 million voters.
She said she had always honoured and upheld the country's Constitution.
She also reminded Senators of her past as a resistance fighter who opposed military rule.
She said that even when she was tortured she continued to fight.
Her fight, she said, had been for a more equal society and that that the achievements of her government in that field were now "at risk".
She added that she was determined to continue her fight against the attacks against her, which she said amounted to a "coup".
Ms Rousseff said she had been "unjustly accused" of crimes she said she had not committed.
"I can't help but taste the bitterness of injustice," she said.
She also warned of the dangers she said the interim government of acting President Michel Temer posed.
Ms Rousseff said Mr Temer's administration would limit public spending and act in the interest of a small economic elite.
After giving her defence, she will be questioned by senators.
When Ms Rousseff arrived at the Senate building in Brasilia shortly after 09:00 local time (12:00 GMT), she was accompanied by her friend and mentor, former President Luiz Inacio Lula da Silva.
She greeted supporters before entering the Senate smiling.
A group of about 200 people had gathered outside the building chanting: "Come back Dilma!"
The impeachment vote is scheduled for Tuesday but analysts say it could slip into Wednesday.
For her to be removed from the presidency permanently, 54 of the 81 senators would have to vote for her impeachment.
Brazilian daily Folha de Sao Paulo says it has spoken to all the senators ahead of the vote and that 52 have so far declared themselves in favour of the impeachment.
Eighteen told the newspaper they were opposed to the impeachment and 11 either did not say which way they would vote or were undecided.
If Ms Rousseff, 68, is impeached, acting President Michel Temer will serve out her term, which ends in December 2018.
Mr Temer, who was Ms Rousseff's vice-president, assumed the role of acting president in May when Ms Rousseff was suspended from office pending the impeachment trial.
On Sunday, a few hundred supporters of Ms Rousseff demonstrated in the capital, Brasilia, against her impeachment and called for the removal of Mr Temer.
But in the past months there have also been large rallies against Ms Rousseff and against corruption in politics in general.
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