"The fight continues," crowds chanted as about 200 people gathered in Bujumbura's Musaga district on the second day of demonstrations. Protesters massed in other parts of the city and tires burned in the streets.
Police slapped and kicked prominent activist Pierre Claver Mbonimpa as they arrested him after he appeared on a live radio program, according to a Reuters witness. Officers then raided and shut down the Radio Broadcasters' Association studio that had broadcast the show.
President Pierre Nkurunziza's announcement on Saturday that he would run in the June 26 election triggered unrest in the east African nation that emerged from an ethnically fueled civil war in 2005.
Activists say Nkurunziza broke the constitution and the Arusha peace agreement that ended the civil war, both documents which limit the president to two five-year terms.
Nkurunziza's supporters say his first term does not count as he was picked by lawmakers, not elected.
The tensions have sent thousands of people fleeing across the border to Rwanda and created fresh turmoil in a region where other presidents, including Joseph Kabila in Democratic Republic of Congo, are nearing presidential term limits.
A Reuters witness said the army had been deployed on the streets and now outnumbered police.
One army officer was seen stopping police firing tear gas. "Don't use violence. If anything worse happens, you will be responsible for that," the military officer said.
Protesters blocked roads with burning tires and large stones, while police appeared to be trying to keep protesters contained in different neighborhoods to stop them converging on the city center.
Five Death on Sunday
Hours before his arrest, Mbonimpa told Reuters at least five people were killed in the capital on Sunday, three of them in protests and two more in an attack by the ruling party's Imbonerakure youth wing. Earlier reports had said two people died.
“The military are aware that we are going to hold protests, but have warned us that they should remain peaceful and that’s all we are asking for," he said.
The ruling CNDD-FDD party has repeatedly denied charges its youth wing is armed and trying to cause violence. The head of police was expected to hold a news conference later in the day.
The police had no immediate comment on any casualties.
Diplomats and opponents say the police are seen as more aligned to the ruling party, a charge the party denies.
African and Western nations had all pressed Nkurunziza not to run again. The U.S. State Department said it was disappointed by the president's decision and said it would take "targeted measures" against anyone instigating or taking part in violence.
Bob Rugurika, another activist and director of private Burundi radio station RPA, said his station and two others had been stopped from broadcasting in the countryside, where much of Nkurunziza's popular support is based.
Rwandan officials said more than 20,000 people had now fled from Burundi to Rwanda, where more than 800,000 mostly Tutsis and moderate Hutus were killed in a 1994 genocide. Thousands have also fled to neighboring Congo.
Burundi's civil war pitted the army, then dominated by the ethnic Tutsi minority, against rebel groups mostly made up of majority Hutus, one of them led by Nkurunziza. The army now includes both ethnic groups.