Protesters in Paraguay stormed and set fire to the country's Congress after a bill was signed allowing the president to seek re-election.
The country's constitution, passed in 1992 after 35 years of dictatorship, limits the president to a single five-year term.
However on Friday the senate secretly voted for an amendment that would allow President Horacio Cartes to run for re-election.
Senator Desiree Masi, from the opposition Progressive Democratic Party, said: "A coup has been carried out.
"We will resist and we invite the people to resist with us."
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 Asuncion and elsewhere in the country well into the night, according to local media.
Earlier, television images showed protesters breaking windows of the Congress and clashing with police, burning tires and removing parts of fences around the building. Police in riot gear fired tear gas and rubber bullets.
Several politicians and journalists were reportedly injured, and Interior Minister Tadeo Rojas said several police were hurt. One member of the lower house of Congress, who had been participating in protests that afternoon, underwent surgery after being hit by rubber bullets.
The number of casualties was unknown.
Cartes called for calm and a rejection of violence in a statement released on Twitter.
"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, tranquility and general wellbeing of the Paraguayan people."
The Office of the United Nations High Commissioner for Human Rights said it was monitoring the events.
"I call on political leaders to avoid inciting violence and seek dialogue," the commission's regional representative for South America, Amerigo Incalcaterra, said in a statement.
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.