Bujumbura - Burundi has charged 65 protestors with rebellion after they were arrested during clashes with police while calling for the president not to seek re-election, a prosecutor said.
Tensions are high with opposition parties concerned at incumbent President Pierre Nkurunziza's expected bid for re-election in June despite the constitutional limit of two terms.
Nkurunziza has not yet confirmed whether he intends to try stay in power, but his supporters say he has every right to do so.
Police arrested scores of protestors on Friday, and on Saturday charged 65 of them with "participation in an insurrectionary movement", state prosecutor Arcade Nimubona said late Saturday.
Some stones were thrown at police, who stopped protests using tear gas and water cannon.
Those charged face a possible life sentence if found guilty of armed insurrection.
In April, a court handed life sentences to 22 young activists for "armed insurrection" following violent clashes with the police in Bujumbura.
Rights activist Pierre-Claver Mbonimpa, president of Burundi's influential campaign group Aprodeh, condemned the charges.
"It is political issue, because these people were about to exercise a right under the constitution" to protest Mbonimpa said.
A large crowd of opposition activists gathered to sing songs outside court to support those who were charged.
"All this is done to discourage us, but we want to tell Nkurunziza we will not give up, we will continue to protest until he says he will not run," said one protestor, asking not to be named.
Burundi, a small landlocked nation in central Africa's Great Lakes region that emerged in 2006 from a brutal 13-year civil war, holds general elections in May to elect lawmakers before a presidential poll in June.
The UN Security Council on Friday warned that the upcoming elections could turn violent and vowed to take action against those who are fomenting unrest.
Chauvineau Mugwengezo, president of the opposition UPD party, said protests would continue.
"We know the powers are ready to kill us, to imprison us, but we are ready to lose our blood rather than give up this fight," said Mugwengezo.
Five opposition parties, as well as splinter groups of the ruling CNDD-FDD who oppose Nkurunziza, have launched a campaign calling for the president not to run again.
The CNDD-FDD is expected to hold a party congress by the end April to nominate its presidential candidate in the polls.