At least five people were killed in overnight clashes in Burundi, police said on Sunday, and residents reported a battle at the president's office in the capital Bujumbura which has been plagued by violence since a disputed presidential election.
Burundi has been in crisis since President Pierre Nkurunziza announced in April he planned to run for a third term - a move opponents said violated the constitution and a peace treaty that ended a 12-year civil war in 2005.
Hundreds have been killed in related violence since April and 217,000 people have fled to surrounding countries, raising fears of a slide into ethnic conflict in a region where memories of the 1994 genocide in neighbour Rwanda are still raw.
Nkurunziza said a court ruling allowed his bid and went on to win a disputed election in July.
Residents in Bujumbura's Rohero neighbourhood, close to the president's office, said they heard shooting and explosions overnight.
"We heard lot of shootings, explosions and shelling. And it was a kind of exchange of fire between soldiers at the presidency and attackers. Mortars were also fired," a night watchman near the president's office said.
Officials and police could not be reached for comment and state media television and radio have not reported any violence at the presidency.
However, Bujumbura Mayor Freddy Mbonimpa said four people had been killed across the capital, two policemen wounded and 28 people arrested.
The arrests occurred in Bujumbura's Ngagara neighbourhood on Saturday afternoon when police raided a bar and detained 28 youths on suspicion of having weapons.
"That is why they came to arrest them for investigation which have now begun," Mbonimpa said. The mayor lives in the neighbourhood and survived a gun attack at his house earlier this week.
But residents said the youths were unarmed, and they were old schoolfriends gathering before a wedding. Later that evening, protesters took to the street, shouting: "Nkurunziza out, we will never accept your illegal third term."
This was followed by the sound of gunfire and explosions, according to a resident.
Blasts, and grenade and gun attacks in Bujumbura have become frequent. The leader of a failed coup in May, General Leonard Ngendakumana, has said his loyalists were behind some of the assaults that targeted police, who opponents blamed for shooting and killing demonstrators.
The general has said that the only way to get Nkurunziza to end his campaign for a third term is to use force. Rights groups have accused the government of using violence to stifle dissent.
Police in Kirundo, a province bordering Rwanda, said one person was killed in a shootout between a gunman and police at a bar.
"We registered a collateral victim in the exchange of fire between police officers and criminals," said one police officer in Kirundo, asking not to be named. One of the gunmen was arrested, another policeman said, without giving further details.
Last week, the United Nations Security Council asked Secretary-General Ban Ki-moon to report within 15 days on options for boosting the U.N. presence in Burundi.