Demonstrators in the Paraguayan capital Asuncion stormed the National Congress building in a protest against senators' closed-door vote to allow President Horacio Cartes to run for re-election.

The opposition crowd that deemed the measure illegal was on Friday confronted by police wielding water hoses. They stormed the legislature, broke windows and fences.

Police fired rubber bullets to disperse them, RT News reported.

According to photos posted on social media, demonstrators managed to storm the first floor of the Congress after breaching the poorly armed police lines.

The rioters then set fire to papers and chairs causing the flames to spread throughout the floor.

The building was eventually cordoned off by police after the entire ground floor burned down.

According to media reports, some MP's were still inside the building.

"We guarantee that the police will not repress [you] again. We ask you not to break down the fences, not to try to enter the Congress building," Police Commander Crisis Sotelo said in a desperate plea to the rioters.

"We ask for calm, tranquility," he said.

Several politicians, including Senate Speaker Roberto Acevedo and journalists were among the wounded.

"The police were not under any instruction to repress. They were attacked," Interior Minister Tadeo Rojas said, blaming rioters for attacking the building and the police.

Local media said several people were hurt by rubber bullets. Among those were former Interior Minister Rafael Filizzola of the opposition Popular Democratic Party, and Efrain Alegre, the presidential candidate from Authentic Radical Liberal Party.

Paraguay's constitution allows presidents to serve only a single term in office to guard against a return to dictatorship after Alfredo Stroessner ruled from 1954 until 1989, the BBC reported.

But an agreement between the ruling and opposition party approved the amendment to allow Horacio Cartes of the Colorado Party to run for re-election in 2018.

The reform will also allow left-wing former President Fernando Lugo, who held power from 2008 to 2012, to run for office again.

Before the amendment is cemented, it must be approved by the Chamber of Deputies, where 44 of the 80 members belong to the ruling Colorado Party.

The vote will take place early on Saturday, according to a document posted on the lower chamber's official Twitter account.

Those opposing the amendment have promised to resist the move, calling the change to the constitution a "coup d'etat" and imposition of "dictatorship".