Burundi charged the leader of a key opposition party and 71 supporters with rebellion Wednesday after violent clashes last week, a prosecutor said, amid worsening tensions in the small central African nation.

Party leader Alexis Sinduhije, who is on the run from police, and the other activists face life in jail if found guilty.

State prosecutor Arcade Nimubona told AFP the activists from the Movement for Solidarity and Development (MSD) party were arrested Saturday after large numbers took part in a "group jog" to the centre of the capital Bujumbura, where they were dispersed by riot police firing tear gas.

An AFP reporter said some 200 people sought refuge at the party headquarters, taking two officers hostage, before police stormed the building.

Clashes lasted for over an hour, with at least 20 activists and five policemen wounded.

Nimubona said that 71 of those arrested after the clashes had been charged with "rebellion, insult and violence towards the custodians of public order, and the participation in an armed insurrection."

An arrest warrant has been issued for Sinduhije for his "participation in an armed insurrection as a leader of the movement," said Nimubona, adding he hoped trials would begin as "quickly as possible".

'Dangerous escalation' of tensions

Sinduhije, a former journalist and founder of one of the most listened to private radio stations RPA, only returned to Burundi last year after fleeing violence following elections in 2010.

His MSD party, founded in 2009, is supported largely by the youth, but like most of Burundi's opposition parties, it boycotted the presidential and legislative elections the following year.

The United Nations in Burundi on Monday expressed concern about the "radicalisation" of both the opposition and government, calling for restraint and dialogue ahead of elections scheduled for 2015, in which President Pierre Nkurunziza is expected to campaign for a third term in office.

The US State Department has also condemned both the excessive use of force against the opposition and the taking of police officers as hostages by the activists.

READ: US condemns 'heavy-handed tactics' by Burundi police in clashes

Tensions between the ruling party CNDD-FDD of President Pierre Nkurunziza and opposition parties have grown in recent weeks.

Last month leaders of the opposition Uprona party were arrested, which threatened to upset a delicate power-sharing arrangement between Burundi's majority Hutu and minority Tutsi communities, who are still struggling to reconcile after decades of conflict.

READ: What will be the effect of the political falling out in Burundi?

Burundi's history is marred by bitter ethnic killings and civil war.

Tensions remain high, with rebel gunmen launching a series of attacks since the 2010 boycott of General Election.

Civil society leader Pacifique Nininahazwe said the arrests and charges against the MSD party members had made him "fear for the future", adding that both government and the opposition are locked into a pattern of confrontation.

"It does not bode well for the 2015 elections, and even peace in this country," he told AFP.

"If nothing is done, if the international community does not put pressure on both sides, including for there to be an opening up of the political arena in this country, the general elections in 2015 will be seriously off track," he added.

A diplomat in Bujumbura, speaking on condition of anonymity, warned of a" dangerous escalation" of tensions in the country.