Hundreds of landless farmworkers armed with sticks and stones invaded Brazil's Congress on Tuesday, smashing windows and furniture to demand agrarian reform.

The hour-long melee left injured 20 people — both demonstrators and congressional security guards. One guard suffered a fractured skull.

About 300 members of the Movement for the Liberation of the Landless, or MLST, waved red flags and held up banners saying "Stop Criminalizing Social Movements."

Protesters also overturned and smashed a red car outside the building that was being raffled among congressional workers.

CountryWatch: Brazil

Hundreds of demonstrators were detained and taken by bus to the Brasilia's Nilson Nelson Sports Gymnasium, the police department said without providing precise numbers

According to government news service Agencia Brasil those detained could be charged with assault with intent to commit murder, trespassing, aggravated battery and property damage.

The presidential press office issued a statement describing the incident as an "act of vandalism" that "violates the principles of democracy."

Aldo Rebelo, president of the Chamber of Deputies, Brazil's lower house, said Congress had summoned security guards to arrest the vandals and called the army to clear out the invaders. But after the protesters agreed to leave peacefully he canceled the request for police and troops.

The MLST is a splinter group of the better known Landless Rural Workers' Movement, or MST, which for years has conducted high-profile, organized invasions of land it deems unproductive.

Under Brazil's 1988 Constitution, unproductive land may be expropriated as long as the owner is compensated.

MLST leader Marcos Praxedes said the demonstrators wanted to enter the congressional building peacefully to demand a speedier land reform process.

"But security agents blocked us and then attacked us," he said. "And we defended ourselves."