Authorities in Burundi arrested a leading dissident and shut down the main independent radio station Monday as they battled a second day of demonstrations against a bid by the president to cling to power for a third term.

The army was also deployed around the capital Bujumbura, after at least two people were shot dead during clashes with police in the capital Bujumbura and after two further deaths were reported overnight in alleged attacks by ruling party militia.

The unrest erupted on Sunday after the ruling CNDD-FDD party, which has been accused of intimidating opponents, designated President Pierre Nkurunziza its candidate in the June 26 presidential election.

A senior police official said at least 320 people have been arrested in the unrest.

The president, a former rebel leader and born-again Christian, has been in power since 2005. Opposition figures and rights groups say his attempt to stay put goes against the constitution as well as the peace deal that ended a civil war in 2006.

Hundreds of thousands of people were killed in the 13-year conflict, and there are fears the upsurge in political tensions could plunge the country back into violence.

On Monday demonstrators were back on the streets, with police using tear gas in Cibitoke, in the north of Bujumbura, to prevent around 1,000 demonstrators reaching the centre. Several other demonstrations were reported across the capital.

Leading human rights activist Pierre-Claver Mbonimpa was also arrested: a witness, who asked not to be named, said Mbonimpa was arrested "brutally" during a police raid on the headquarters of a media association.

Mbonimpa's lawyer, Armel Niyongere, said he had not been informed of the charges against his client but believed "the arrest is linked to his call for demonstrations today".

An arrest warrant has also been issued for Vital Nshimirimana, head of a prominent NGO forum.

"This is only the beginning. The movement will not stop until Nkurunziza announces he is no longer candidate," Nshimirimana, who has gone into hiding, told AFP in a telephone interview.

"The Burundian people and international community are witness to the fact that our protests are peaceful," he said, condemning the violent crackdown.

- Radio station shut down -

The government has banned all protests, and on Monday also shut down Burundi's main independent radio station.

"The radio is off the air after a decision by the authorities," said Gilbert Niyonkuru, head of programming at the influential African Public Radio (RPA), which has for months been reporting on government intimidation of opponents.

Broadcasts by the station's studios outside of the capital Burundi have also been halted, with station officials saying they had been accused of "complicity and participation in an insurrectional movement."

Relatives of Sunday's slain demonstrators told AFP that they were shot at close range by police.

But speaking on state television, Bujumbura's mayor Saidi Juma claimed the pair were killed by other demonstrators.

Two further deaths during overnight violence were confirmed by the army.

At least 15,000 Burundians have fled the country to neighbouring Rwanda in recent weeks, according to the UN's refugee agency, which has warned that those numbers could rise.

Many are fleeing threats by the pro-government militia Imbonerakure, the youth wing of the ruling CNDD-FDD party. Rights groups allege that the militia has been armed and trained over the past year in order to help Nkurunziza remain in office.

The US embassy in Djibouti said it was watching the situation closely and warned it would "hold accountable those responsible for violence against the civilian population".

After Sunday's protest deaths, the African Union appealed to Burundi's government to "exercise the highest restraint and protect the population".

The influential Catholic Church has also spoken out against the president's plans to stay put.

UN rights chief Zeid Ra'ad Al Hussein warned recently the country was at a "crossroads" between a fair vote and a route back to its "horrendously violent past".