At least 12 gunmen have died in battle with Burundi security forces following assaults on two military bases on Friday, in the worst fighting since the failed coup in May, an army spokesman said.

"21 armed young men have also been arrested after attacking the two army bases in Bujumbura in efforts to loot weapons in order to liberate Mpimba prisoners," said Col Gaspard Baratuza.

Dozens of protestors arrested since April when unrest erupted, after President Pierre Nkurunziza decided to seek a third term that he won in disputed July polls, have been held at the Mpimba prison.

Heavy gun fire and explosions that shattered the calm in Bujumbura overnight intensified Friday morning and spread to other parts of the capital.

“This is quite different from night shootings which was only based on one side of the capital,” one resident reported.

Armed men attacked the Ngagara and Musaga military camps at around 3am (0500 GMT).

"I saw ambulances going in and out of the Ngagara army base, it is tense here we don't know what is going to happen," another resident said.

An officer who spoke to AFP said the army also had some casualties.

President Pierre Nkurunziza’s advisor Willy Nyamitwe accused Sindumuja (I am not a slave) protesters – who were mobilised by civil society and the opposition in the run-up to President Nkurunziza’s election for a controversial third term - of trying to release prisoners from Mpimba central prison.

“The attack on military camps was a diversion,” he said on Twitter.

Security forces have blocked the main roads to Bujumbura, forcing workers to return home for lack of access to duty stations.

READ: Kenya Airways, RwandAir cancels flights to Burundi after unrest

With all businesses paralysed in the capital, even students couldn't make there way to school as shootings and shelling continue to intensify.