Opposition leaders also said they were suspending talks with the government of President Pierre Nkurunziza after Zedi Feruzi, the leader of the Union for Peace and Development (UPD), a small opposition party, was gunned down in the capital yesterday.
"Those who killed Feruzi Zedi will pay sooner or later," one sign held by mourners read.
The killing came a day after a grenade attack on a busy market killed three people and injured around 40 others.
The crisis also sparked a failed coup against President Nkurunziza last week, and there are fears Burundi, which only emerged from a 13-year civil war in 2006, could be plunged back into conflict.
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Condemning the apparent assassination as "an awful act", activists said in a statement they were "suspending participation in dialogue with the government" that had been supported by the United Nations and African Union.
They also said the murder could have been part of an alleged "plan to physically eliminate" leaders of the campaign against Nkurunziza. A journalist who witnessed the attack said the gunmen were clad in uniforms similar to those worn by the presidential guard.
While the streets of the capital were calm today in line with a weekend truce, protesters were readying to resume the demonstrations.
Civil society leader Pacifique Nininahazwe called the truce "to allow the people to bury with dignity those who died for democracy," but warned the "protests will resume on Monday with even more force.