The head of the main opposition party said an activist had been shot dead.
Paraguay’s 1992 constitution, introduced after 35 years of dictatorship, limits the president to a single five-year term.
However, sitting President Horacio Cartes is trying to remove the restriction and run for re-election.
Demonstrators were photographed smashing in windows of the congress building in Asuncion on March 31 and setting fire to the interior.
According to the AFP, protesters “ransacked” the offices of those who backed the bill.
Police used mounted units, rubber bullets, and water cannons to disperse the crowd. Local media reports said dozens of people had been injured, including protesters, politicians, and police officers.
The head of Paraguay’s opposition Liberal party, Efrain Alegre, said a young man had been killed during the protests. Police are yet to confirm this.
In a statement released on Twitter, President Horacio Cartes appealed for calm.
He said: “Democracy is not conquered or defended with violence and you can be sure this government will continue to put its best effort into maintaining order in the republic.”
The attorney general’s office said it had followed the events closely and was investigating the violence.
Earlier, the crowd took to the streets following a private meeting of 25 senators – a slight majority of the house – which approved a bill to amend the constitution.
The bill must also be approved by the other house of parliament – the chamber of deputies – where Horacio Cartes’ party holds a majority.
The chamber’s president, Hugo Velázquez, told ABC Color that the sitting planned for the following morning would no longer take place and no decision would be made on April 1.
Opponents say the bill will weaken Paraguay‘s democratic institutions.
Opposition senator Desiree Masi said: “A coup has been carried out. We will resist and we invite the people to resist with us.”
Paraguay was controlled by military ruler General Alfredo Stroessner, who seized power in a coup, from 1954 until 1989.
The new constitution in 1992 created the modern government, but there has been a long period of political instability and party infighting, as well as a failed coup attempt.
President Horacio Cartes’ term is due to end in 2018.
The change, if approved, would also allow former president Fernando Lugo to run again.
Fernando Lugo was ousted in 2012 over his handling of a land eviction in which 17 people were killed.
The US House Ethics Committee has voted to release its report on former Republican Representative…
ABC News has agreed to pay $15 million to President-elect Donald Trump to settle a…
South Korea’s parliament has voted to impeach President Yoon Suk Yeol over his failed attempt…
Israeli war planes have carried out more than 100 air strikes in Syria on December…
President-elect Donald Trump has threatened to impose 100% tariffs on the BRICS countries if they…
Syrian troops have withdrawn from the city of Aleppo following an offensive by rebels opposed…