Authorities said 30 people, including some opposition leaders, were injured in unrest in the capital after senators approved the bill in a secretive vote.
Opposition leaders denounced the vote Friday as a "parliamentary coup," saying it could clear the way for a return to dictatorship in the landlocked South American nation of 6.8 million people.
Furious protesters broke into the Congress late Friday, ransacking lawmakers' offices and starting fires after senators approved a proposal to allow the president to run for re-election.
Right-wing President Horacio Cartes is seeking to amend the constitution to enable himself to run for office again in 2018 after his current term ends.
The measure requires approval in the lower house, the Chamber of Deputies, which delayed a vote originally set for Saturday.
Riots, fires, arrests
Rodrigo Quintana, 25, leader of the opposition Liberal Party's youth branch, was shot and killed as police searched the party's premises in Asuncion, party leader Efrain Alegre said.
The interior ministry said in a statement that "the authorities are investigating the circumstances of the death, which is presumed to have occurred at the hands of a member of the National Police."
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It added: "We are going to establish fully what happened, and those responsible will be brought to justice."
Police raided the party offices after activists took refuge there during a night of riots, Alegre said.
The injured included three lawmakers, according to firefighters and an opposition senator. Police said 211 people were arrested, some of them minors.
To chants of "Dictatorship never again!" hundreds of protesters clashed with riot police, who used mounted units and water cannon to disperse them.
Calm was restored around 0400 GMT Saturday. Large numbers of police remained on alert.
Rubber bullets
Cartes's allies in the upper house of the legislature passed the bill on Friday, sidestepping resistance from opponents.
The vote took place in Senate offices as the main assembly hall was occupied by senators from the Liberal Party, opposed to the reforms.
Opposition senator Luis Wagner said those injured included Senate speaker Roberto Acevedo, lawmaker Edgar Ortiz, who was hit in the mouth by a rubber bullet fired by police, and Liberal leader Alegre, who lost to Cartes in the 2013 presidential elections.
Watch: Protesters storm Paraguay's Congress
Acevedo has challenged the bill in the Supreme Court, arguing it is unconstitutional.
The measure was scheduled to be considered Saturday in the Chamber of Deputies, where the president has a majority.
But after the rioting, the president of the lower house, Hugo Velazquez, announced the vote was postponed, saying he was shocked by the violence.
"I hope that calm and harmony will return," Velazquez said in a televised message.
Paraguay president fires top aides
Paraguay's president sacked his interior minister and police chief Saturday, following a clash that led to the death of a young activist as rioters angry about electoral reform stormed Congress.
Closed circuit cameras captured the death of 25-year-old Rodrigo Quintana, leader of the opposition Liberal Party's youth branch, who was apparently shot by police early Saturday as they searched the party's offices in Asuncion for protesters.
About 30 people were injured, including three lawmakers, according to firefighters and an opposition senator. Police said 211 people were arrested, some of them minors.
President Horacio Cartes on Saturday responded by firing Interior Minister Tado Rojas and Police Commissioner Crispulo Sotelo.
But Foreign Minister Eladio Loizaga slammed the unrest as "the work of a violent group that does not understand what democracy is, does not understand tolerance."
"Paraguay's democracy is stronger than ever," he said, adding that security forces acted with "total responsibility."
Furious protesters broke into the Congress late Friday, ransacking lawmakers' offices and starting fires after senators approved a proposal to allow the president to run for reelection.
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Opposition leaders denounced the secretive vote Friday as a "congressional coup," saying it could clear the way for a return to dictatorship in the landlocked South American nation of 6.8 million people.
Right-wing leader Cartes is seeking to amend the constitution to enable himself to run for office again in 2018 after his current term ends.
He blamed the violence on "a group of Paraguayans embedded in politics and the media aimed at destroying democracy and political and economic stability."
"Democracy is not won or defended by violence," he said on Twitter.
"We must not allow barbarians to destroy the peace, tranquility and welfare of the people."
Police on alert
To chants of "Dictatorship never again!" hundreds of protesters clashed late Friday with riot police, who used mounted units and water cannon to disperse them.
Police raided the Liberal Party offices after activists took refuge there during a night of riots, party leader Efrain Alegre said.
Calm was restored around 0400 GMT Saturday, though police remained on alert.
Paraguay has banned presidents from re-election since 1992 to avoid a return to dictatorships like that of General Alfredo Stroessner, who ruled from 1954 to 1989.
Removing the ban would also allow left-wing former president Fernando Lugo to run again. He held power from 2008 to 2012, when he was removed after an impeachment trial.
The measure was scheduled to be considered Saturday in the Chamber of Deputies, where the president has a majority.
But after the rioting, the president of the lower house, Hugo Velazquez, announced the vote was postponed, saying he was shocked by the violence.
"I hope that calm and harmony will return," Velazquez said in a televised message.
If the latest measure is approved by the two houses, it is expected to be put to a referendum within three months.
History of dictatorship
Paraguay has banned presidents from re-election since 1992 to avoid a return to dictatorships like that of General Alfredo Stroessner, who ruled from 1954 to 1989.
Removing the ban would also allow left-wing former president Fernando Lugo to run again. He held power from 2008 to 2012, when he was removed after an impeachment trial.
If the latest measure is approved by the two houses, it is expected to be put to a referendum within three months.
The opposition condemned the move as a "parliamentary coup" and called for resistance.
"It is a dictatorial plan by Horacio Cartes with the complicity of Ferdinand Lugo," said Senator Carlos Amarilla.
'Barbarians'
Cartes blamed the violence on "a group of Paraguayans embedded in politics and the media aimed at destroying democracy and political and economic stability."
"Democracy is not won or defended by violence," he said on his Twitter account.
"We must not allow barbarians to destroy the peace, tranquility and welfare of the people."
