Protesters stormed and set fire to Paraguay's Congress on Friday, March 31 after the Senate secretly voted for a constitutional amendment that would allow President Horacio Cartes to run for re-election, Reuters reports.

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 Asuncion and elsewhere in the country well into the night, media reported.

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 injured, media reported, 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 unrest coincides with a rare high-level international event in the landlocked South American country. Thousands of businessmen and government officials descended on Asuncion this week for the Inter-American Development Bank's annual board of governors meeting.

While Paraguay long suffered from political uncertainty, the soy and beef-exporting nation has been attracting investment in agriculture and manufacturing sectors in recent years as Cartes offered tax breaks to foreign investors.

Instability in the country of 6.8 million is a concern for its much larger neighbors Brazil and Argentina.

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.