Kefa Nibizi, president of the Council for Democracy and Sustainable Development in Burundi (CODEBU), was being held in Bujumbura's central prison accused of "undermining the internal security of the state."
On Friday, the party posted a critical comment about the government on X, formerly Twitter, to mark the 62nd anniversary of the assassination of Burundian independence hero Prince Louis Rwagasore.
"At a time when Burundi is languishing in unprecedented misery because of the failing leadership, the CODEBU party invites the population not to give in to resignation and to follow the example of Prince Louis Rwagasore, to redress the situation which is only getting worse," it said.
In a series of posts on Tuesday, the party voiced "regret" that its comment had not been understood as intended and sought to clarify the message.
CODEBU deputy president Jacqueline Hatungimana called for Nibizi's release, saying the comments did not justify his incarceration.
"We are surprised, we don't understand how he was imprisoned just after being questioned for saying that there are some shortcomings at the current level of leadership," Hatungimana told reporters.
Nibizi's arrest comes as Burundi's sacked former prime minister Alain-Guillaume Bunyoni is on trial accused of undermining national security and insulting the president, although the two cases are not linked.
On Monday, prosecutors also laid another charge against Bunyoni of "attempting to assassinate the head of state" as they opposed his bid for bail, according to sources close to the case.
Since coming to power in 2020, Burundi's President Evariste Ndayishimiye has been hailed for gradually ending years of isolation under his predecessor Pierre Nkurunziza's chaotic and bloody rule.
But he has failed to improve a dire record on human rights and political freedoms, and Burundi also remains one of the poorest countries on the planet.
Last week, the UN Human Rights Council renewed the mandate of a special rapporteur on Burundi's rights situation for another year and urged the government to fully cooperate with him.
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