Nairobi - Burundian President Pierre Nkurunziza has "declared war" on his people by clinging onto power, key opposition leader Leonard Nyangoma said on Thursday, demanding he step down to allow a "political transition."Nyangoma, a veteran political leader in Burundi who heads the opposition CNARED coalition, demanded the "immediate departure" of the president.Nkurunziza's successful effort to bulldoze his way into a third term in a controversial July election sparked protests and a failed coup, with a sweeping crackdown and an exodus of citizens fleeing the unrest.When his candidacy was announced in April, it was condemned as unconstitutional by the opposition and sparked months of protests."The regime in Bujumbura is desperate to stay in power, he has declared war on the people," Nyangoma told AFP, urging people to remain "calm and united".There are widespread fears - both inside and outside Burundi - that the tiny country in the heart of central Africa's troubled Great Lakes region could be plunged back into civil war.Ethnic divisionsAt least 300 000 people were killed in the civil war which ran from 1993-2006."The situation is extremely serious: the political, socio-economic situation is deteriorating day by day," Nyangoma added, speaking from Europe, where he is now based.Security forces are targeting opponents of Nkurunziza with "numerous arrests, torture, killings and atrocities," he added.Nyangoma was a founding member of the ruling CNDD-FDD party of Nkurunziza.He now heads a coalition that brings together leading opposition politicians, exiled opponents and civil society leaders, with the aim of creating a common front against the president.The small, landlocked country is one of the world's poorest nations.Nkurunziza, a 51-year-old former sports teacher and born-again Christian, was a Hutu rebel leader during the civil war.That war pitched rebels from the majority Hutu people against an army dominated by the minority Tutsi.But despite fears of renewed Hutu-Tutsi tensions, Nyangoma said people had not reacted to those who tried to stoke ethnic divisions."Since the beginning of the crisis, the regime tried to play the ethnic card, but it did not work," he said.