Protesters stormed Paraguay's Congress on Friday and set fire to it after the Senate secretly voted for a constitutional amendment that would allow President Horacio Cartes to run for re-election.
The country's constitution has prohibited re-election since it was passed in 1992 after a brutal dictatorship fell in 1989.
Right-wing President Cartes is seeking to reform the constitution to let him run for office again in an election in 2018 after his current term ends.
His allies in the upper house of the legislature passed the bill, sidestepping resistance from opponents, who say the bill clears the way for dictatorships.
"A coup has been carried out. We will resist, and we invite the people to resist with us," said Senator Desiree Masi from the opposition Progressive Democratic Party.
Television images showed protesters breaking windows of the South American country's Congress building after several hours of escalating violence and confrontations with police.
Demonstrators burned tyres and removed parts of the fences surrounding the Congress building, and police in riot gear responded by lobbing tear gas and firing rubber bullets.
Several politicians and journalists were injured, local media reported, and Interior Minister Tadeo Rojas said many police were hurt.
The Senate voted during a special session in a closed office in Congress rather than on the Senate floor.
Twenty-five lawmakers voted for the measure, two more than the 23 required for passage in the 45-member upper chamber.
The proposal goes to the House, where it appeared to have strong support. A vote expected to take place early on Saturday was called off until the situation calmed down.
Several Latin American countries, including Paraguay, Peru and Chile, prevent presidents from running for consecutive terms in a region where the memories of military dictatorships remain ripe.
Others, including Colombia and Venezuela, have changed their constitutions to give sitting presidents a chance at re-election.
Paraguay's measure would apply to future presidents as well as Cartes, a soft-drink and tobacco mogul elected to a five-year term in 2013.