Rousseff, 68, was convicted by 61 of the 81 senators of illegally manipulating the national budget. The vote, passing the needed two-thirds majority, meant she was immediately removed from office.
However, in a surprise twist, a separate vote to bar Rousseff from holding any public office for eight years failed to pass, meaning she could in theory re-enter political life.
In a statement from the Alvorada presidential palace on the outskirts of the capital Brasilia, Rousseff, from the leftist Workers’ Party, called her forced exit a “coup.”
“They decided to interrupt the mandate of a president who had committed no crime. They have convicted an innocent person and carried out a parliamentary coup,” she said.
Cheers — and cries of disappointment — erupted in the blue-carpeted, circular Senate chamber as the impeachment verdict flashed up on the electronic voting screen.
Pro-impeachment senators burst into a rendering of the national anthem, some waving Brazilian flags, while leftist allies of Rousseff, who back her claims of being victim of a right-wing coup, stood stony-faced.
“I will not associate my name with this infamy,” read a sign held up by one senator.
Renan Calheiros, the Senate president, rejected Rousseff’s coup claim, saying her impeachment might not have been perfect, but had “the DNA of democracy, the DNA of the constitution.”
Rousseff’s vice president turned bitter political enemy, Michel Temer, was to be sworn in as her replacement at 4:00 pm (1900 GMT), Calheiros announced.
Temer, a veteran center-right politician, was then to leave for a G20 summit in China.
– Anti-Rousseff anger –
Rousseff, Brazil’s first woman president, stood accused of taking illegal state loans to patch budget holes in 2014, masking the country’s problems as it slid into its deepest recession in decades.
She told the Senate during a marathon 14-hour session on Monday that she is innocent and that abuse of the impeachment process put Brazil’s democracy, restored in 1985 after a two-decades-long military dictatorship, at risk.
Recalling how she was tortured and imprisoned in the 1970s for belonging to a leftist guerrilla group, Rousseff urged senators to “vote against impeachment, vote for democracy… Do not accept a coup.”
However, huge anti-Rousseff street demonstrations over the last year have reflected nationwide anger at her management of a country suffering double-digit unemployment and inflation.
The once mighty Workers’ Party, meanwhile, has struggled to stage more than small rallies.
Temer, who was in an uncomfortable partnership with Rousseff before finally splitting, will be president until the next scheduled elections in late 2018.
The 75-year-old, known more as a backroom wheeler-dealer than street politician, took over in an interim role after Rousseff’s initial suspension in May.
He immediately named a new government with an agenda of shifting Brazil to the right after more than a decade of leftist rule that saw 29 million people lifted from poverty, but became bogged down in corruption and the economic slump.
Temer has earned plaudits from investors. It remains uncertain whether he will have voters’ support to push through the painful austerity reforms he promises.
– Emotions spill over –
About 50 leftist demonstrators gathered outside the presidential palace to show their support for Rousseff.
“We are protesting against the coup and fighting for democracy,” said 61-year-old farmer Orlando Ribeiro.
In the center of the capital, extra security and the closing of avenues near the Senate caused massive traffic jams. Police said they were preparing for large protests later in the day.
On the Senate floor, emotions crackled in the run-up to the vote, then overflowed as senators made final speeches on what both sides of the debate agreed was history in the making.
Senator Aecio Neves, Rousseff’s narrowly defeated center-right opponent in her 2014 re-election, pronounced triumphantly: “The constitution won. Brazil won!”
But Senator Vanessa Grazziotin was scathing in her summing up of an “illegal process that is called impeachment but is a coup.”
“Temer does not have legitimacy to govern this country,” she warned.
“This is a farce, a farce, a farce,” shouted another pro-Rousseff senator, Lindbergh Farias.
Shaking his fists at the majority backing impeachment, he cried: “They’re going in the garbage can of history.”