French journalist Jean-Philippe Remy, Africa bureau chief for French daily Le Monde, and British photojournalist Phil Moore, were arrested on Thursday afternoon during a raid in which 15 others were also held, police said.
Both journalists have covered the region for years winning several awards for their work.
“The two foreigners were arrested in the company of armed criminals,” the security ministry said in a statement.
Police said a mortar, a Kalashnikov rifle and pistols were seized in the raid in Nyakabiga, a Bujumbura suburb and anti-government protest hotspot.
“If there is no evidence against them, they will be released, of course,” police deputy spokesman Moise Nkurunziza said.
French ambassador Gerrit Van Rossum visited the pair on Friday at the headquarters of the National Intelligence Service (SNR), where they had been questioned. France’s foreign minister Laurent Fabius called for their “immediate release.”
– ‘Consummate professionals’ –
In a statement Le Monde demanded the release of the two journalists who are on assignment for the paper.
“They both have valid visas and were merely exercising their professional duties by meeting all concerned parties involved in the current tensions in Burundi,” Le Monde said.
“The Burundian authorities should immediately release French journalist Jean-Philippe Remy and British photographer Phil Moore, unless there is a credible legal basis for detaining them, and guarantee their safety,” said Carina Tertsakian of Human Rights Watch.
The Foreign Correspondents’ Association of East Africa (FCAEA) said it was “extremely concerned about the arrests of our esteemed and dear colleagues.”
Moore, 34, has frequently worked for AFP and other international publications, winning widespread recognition for his photographs of conflicts in eastern Democratic Republic of Congo as well as Libya, Somalia and Syria.
Remy, 49, has won several awards, including the 2013 Prix Bayeux-Calvados for his coverage of the war in Syria.
“We know them to be consummate professionals and are disturbed by news of their detention while they were doing their jobs in Bujumbura,” the FCAEA said in a statement early Friday.
– Burundi to dominate AU talks –
Burundi has been in crisis since April when President Pierre Nkurunziza ran for a controversial third term, sparking street protests, a failed coup, regular killings and a nascent rebellion.
The government has cracked down on the press, forcing independent media to shut down and driving some journalists into exile.
AFP and RFI reporter Esdras Ndikumana, 54, was forced to seek refuge in Kenya in August after he was tortured by the SNR in Bujumbura.
The crisis in Burundi will be top of the agenda this weekend when African leaders meet at their annual summit in Ethiopia, where they are expected to vote on sending 5,000 peacekeepers to Burundi, a plan the government has rejected.
Since Nkurunziza won presidential elections in July, clashes between loyalists and the opposition have turned increasingly violent.
The UN has warned that Burundi risks a repeat of a 1993-2005 civil war, with some 400 dead since April and at least 230,000 people fleeing to neighbouring countries.
On Friday, rights group Amnesty International released satellite photos they said “strongly indicate” five mass graves of those killed during battles in the capital in December.
“These images suggest a deliberate effort by the authorities to cover up the extent of the killings by their security forces and to prevent the full truth from coming out,” Amnesty’s regional chief Muthoni Wanyeki said.