Angry demonstrators in Brasilia have started a fire inside the ministry of agriculture and have damaged several other ministerial buildings.
Brazilian authorities estimate around 35,000 are marching in the capital.
Troops are being deployed to defend government buildings and there is a heavy police presence on the streets.
Protesters are demanding the resignation of president Michel Temer, fresh elections, and for economic reform plans to be withdrawn.
Temer has faced new corruption allegations in the last week, and is facing growing pressure to step down.
According to reports, several ministries are being evacuated because of the protests - but not before the agriculture ministry was damaged.
‘There was an invasion of the ministry's private entrance. They lit a fire in a room, broke photos in a gallery of ex-ministers and confronted police,’ a spokesman told the AFP news agency.
There have been clashes between police and demonstrators and local media report one person has been injured and several have been detained.
Planned demonstrations began peacefully around midday before clashes with police erupted.
Police fired rubber bullets and teargas at some demonstrators, and video footage showed other members of the crowd smashing windows or setting makeshift barriers afire.
Last week, testimony released by the country's supreme court alleged that Temer had taken millions of dollars in bribes since 2010.
The plea-bargain testimony came from bosses of a giant meat-packing firm.
In response, Temer vowed to prove his innocence and remain as president while so doing.
On Saturday, he filed a petition to have the investigation suspended, but reversed that decision on Tuesday.
Temer took office a year ago after his predecessor, president Dilma Rousseff, was impeached.
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