At least 36 people have been killed after unidentified gunmen opened fire at a crowded bar near the Burundi capital, Bujumbura, officials say.
A local hospital is reportedly unable to cope with the wounded, while dead bodies have been left in a car park.
"I heard someone some distance away shout: 'Kill them all,' and they opened fire," one survivor told the BBC.
Burundi's last rebel group officially laid down its arms in 2009 but sporadic attacks have continued.
The BBC's Prime Ndikumagenge in Bujumbura says it is the most deadly attack since last year's disputed poll.
Former rebel leader Agathon Rwasa withdrew from the presidential election and fled the country after his National Liberation Forces (FNL) accused the governing party of fraud.
The government has blamed recent attacks on bandits but our correspondent says some fear a new rebel group has emerged.
There are some reports that the attackers crossed into Gatumba from just across the border in Democratic Republic of Congo.
Presidential trip cancelled
The survivor said the attackers wore military uniforms.
"They really took their time. Two bullets went through my body and another two are still inside. My legs sustained grenade injuries. All I can ask for is peace. I don't know why I should be a victim," he said
A doctor who only gave his name as Leonard told the AFP news agency the hospital where he worked was "totally overwhelmed" by the number of wounded.
"We are lacking blood, equipment and medicine to treat all the injured," he said.
AFP reports that dead bodies had been left in a car park at one hospital.
President Pierre Nkurunziza visited the scene and vowed to bring those responsible to justice.
He said he was cancelling this week's trip to New York, for the UN General Assembly.
Some 300,000 people are said to have been killed in Burundi's 12-year civil war between the minority Tutsi-dominated army and ethnic Hutu rebels.
The conflict officially ended in 2005 with a peace deal which saw former rebel leader Mr Nkurunziza elected president but FNL rebels continued fighting.