Burundi has been hit by a new wave of protests as opposition to President Pierre Nkurunziza's bid for a third term grows.
Gunfire was heard and streets were barricaded in parts of the capital, Bujumbura, in the third day of protests, witnesses told the BBC.
Police are blocking about students in the second city, Gitega, from joining the demonstrations, residents said.
The protests are the biggest in Burundi since the civil war ended in 2005.
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The army and police have been deployed to quell the protests, which have been described by government officials as an insurrection.
'Phone lines cut'
UN chief Ban Ki-moon said, in a statement, that he had despatched his special envoy for the region, Said Djinnit, to Burundi for talks with Mr Nkurunziza.
African Union commission head Nkosazana Dlamini-Zuma said she welcomed a decision by Burundi's Senate to ask the Constitutional Court to rule whether Mr Nkurunziza could stand for re-election.
BBC Burundi analyst Prime Ndikumagenge says the phone lines of private radio stations have been cut, a decision apparently taken by the authorities to prevent news of protests from spreading.
The ruling party's Vice-President Joseph Ntakirutimana has compared one radio station to a former Rwandan broadcaster, accused of fuelling the 1994 genocide.
The Red Cross says at least six people have been killed in the demonstrations since Sunday.
More than 24,000 people have fled Burundi this month, as tensions mount ahead of presidential elections in June, the UN refugee agency said.
This includes 5,000 who crossed into Rwanda at the weekend, it added.
Burundi's ex-President Pierre Buyoya, who was involved in the peace process that ended more than a decade of ethnic conflict, has warned that Burundi could return to war if the crisis is not resolved.
Burundi has a majority Hutu and a minority Tutsi population.
The opposition says Mr Nkurunziza, a former rebel who took power after the civil war ended, should step down.
They say his bid to extend his term is in defiance of the constitution, as it bars the president from running for a third term.
However, Mr Nkurunziza's allies say his first term does not count as he was appointed by parliament and not directly by the people.
More than 300,000 people died in the civil war between the minority Tutsi-dominated army and mainly Hutu rebel groups, including Mr Nkurunziza's CNDD-FDD.