Furious protesters torched Paraguay's parliament building, accusing the country's president of staging a 'coup' over a secret vote allowing him to stand for re-election.
Senators secretly voted for a constitutional amendment allowing President Haracio Cartes to stand for office again.
The move prompted a night of rioting which saw demonstrators storm the Congress building, setting fire to it and injuring a number of police officers.
Several politicians, including the country's former interior minister, Rafael Filizzola, were hit by rubber bullets, the Paraguayan media reports.
In a call for calm, Cartes described the rioters as 'barbarians'.
Journalist Santi Carneri told the BBC that the fire was alight for more than two hours, and described it as a the worst violence since Paraguay became a democracy.
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.'
The country's constitution has prohibited re-election since 1992, when laws were introduced following the overthrow of dictator Alfredo Stroessner in 1989 after a brutal 35 year rule.
Firefighters were eventually able to bring the Congress building fire under control late last night, but protests and riots continued across capital city Asuncion and in other parts of the country.
Television images had earlier shown protesters smashing 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 injured, media reported, and Interior Minister Tadeo Rojas said several police were hurt. 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 rioting broke out as thousands of businessmen and government officials were in the capital for the Inter-American Development Bank's annual board of governors meeting.
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.
CRITICS BRAND SENATE VOTE ILLEGAL AS PROTESTERS RIOT
The Senate voted yesterday 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 law change, 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.