Protesters stormed and set fire to Paraguay's Congress building on Friday 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.
"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.
Firefighters managed to control the flames after protesters left the Congress building late on Friday night. But protests and riots continued in other parts of the capital, Asuncion, and elsewhere in the country well into the night.
Television images showed protesters breaking windows of the Congress building and clashing with police, burning tyres and removing parts of fences around the building.
Police in riot gear fired tear gas and rubber bullets.
TRT World's Christine Pirovolakis reports.
Cartes called for calm and a rejection of violence in a statement.
"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," he said.
"We must not allow a few barbarians to destroy the peace, tranquillity and general wellbeing of the Paraguayan people."
Secret Senate vote
The Senate voted earlier on Friday during a special session in a closed office 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.
Opponents of the measure, who claim it would weaken Paraguay's democratic institutions, said the vote was illegal.
The proposal will also require approval by the House, where it appeared to have strong support. A vote which had been expected early on Saturday was called off until the situation calmed down, said the chamber's president, Hugo Velazquez.
Several Latin American countries, including Paraguay, Peru and Chile, prevent presidents from running for consecutive terms in a region where memories of 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 and Cartes, a soft-drink and tobacco mogul elected to a five-year term in 2013.
His strongest backers want him to be allowed to run for another term, but critics have said a constitutional change aimed at benefiting a sitting president would be unfair.