Peace talks resumed in Tanzania on Saturday in order to end the crisis in Burundi but several opposition figures were not invited to the meeting.
Burundi has been mired in a year-long crisis that has killed more than 450 people since President Pierre Nkurunziza pursued and won a third term. Opponents said his move violated the constitution and a peace deal that ended a civil war in 2005.
Dialogue in Bujumbura last year between the government and opponents failed to bridge differences, and talks mediated by Uganda earlier this year also swiftly stalled.
"The Burundi peace talks began in Arusha today under the facilitation of former Tanzanian President Benjamin Mkapa," Richard Owora Othieno, spokesman of the six-state East African Community, which includes Burundi, said.
The Tanzanian city of Arusha was also the location for negotiations that led to the deal to end the ethnically charged 1993-2005 civil war.
Renewed violence in Burundi has alarmed the region where memories of the Rwanda's 1994 genocide remain etched in memory. Like Rwanda, Burundi has an ethnic Hutu majority and a Tutsi minority.
Till now, the violence in Burundi has largely followed political rather than ethnic loyalties. But diplomats fear ethnic wounds could reopen the longer violence continues.
The Burundian government refuses to sit with key opponents and accuses them of having an involvement in the failed May 2015 coup and also of months of continued violence which included grenade and rocket attacks.
Senior government officials are attending the talks. But opponents said some major opposition groups were excluded, including CNARED, an umbrella group that includes politicians in exile opposed to the third term.
The government has said CNARED is not a registered party.
"For us, there is no dialogue, it is rather a monologue, a time wasting because dialogue or talks should be between two parties in conflict," said CNARED spokesman Pancrace Cimpaye in a statement.
Charles Nditije, president of UPRONA said his party also does not have official status in peace talks.
He also said a grouping of civil society organisations opposed to the third term and other parties were also left out.
The talks were attended by some Western donors and a UN representative. The Tanzanian mediators did not immediately provide a list of opposition groups attending.