Bujumbura - Burundi's police fired tear gas and water cannon at hundreds of protesters on Friday who were calling on the president not to run for a third term, a Reuters witness said.
Supporters have urged President Pierre Nkurunziza to stand in the June presidential race, though he has not commented on his intentions. Opponents say such a move would be unconstitutional.
"We won't let him run again," people in the demonstration in the capital Bujumbura shouted.
Protesters took shelter in nearby shops or buildings before regrouping. Some hurled stones at police.
Opposition parties are discussing uniting behind a single candidate to improve their chances of defeating Nkurunziza, who took office in 2005 after 12 years of civil conflict in the east African nation. He has served two terms.
The constitution and the peace deal that ended that war both say no one should be president for longer than 10 years.
The president's backers argue his first term should not count since he was picked by lawmakers rather than voted in.