Perched on the border with Angola, Congo's two southern Kasai provinces have been relative havens of peace in a country dogged by insecurity eight years after a war that claimed five million lives.

But with local hero Etienne Tshisekedi emerging as the main rival to President Joseph Kabila in a November 28 poll which already has triggered violent clashes, the local calm is at threat.

The 78-year-old Tshisekedi, who returned from self-imposed exile abroad late last year, ratcheted up tensions last week by unilaterally declaring himself president and calling for attacks on prisons to release people he says are political detainees.

The comments were labelled as treasonous by the government in Kinshasa and drew a sharp rebuke from the African Union, which is trying to keep a lid on confrontation ahead of voting.

The United Nations and the European Union joined those warning against bloodshed. The International Criminal Court also vowed last week to investigate perpetrators of election-related crimes, as it is doing in Kenya and Ivory Coast.

Yet Mbuji-Mayi, capital of eastern Kasai, has seen outbreaks of trouble in past weeks. Persistent street rumours -- hotly denied by authorities -- that the vote will be rigged in Kabila's favour are fanning the flames of discontent.

Mainly people here are calm, but when they rise up they don't give up, said Arthur Padingayi of local rights group ASADHO. Alongside neighbouring Katanga, Kasai sought in 1960 to secede, sparking fierce battles with government forces.

In a country where politicians often are seen as serving their own interests, Tshisekedi can claim democratic credentials.

Brought up in western Kasai, he was arrested several times in the 1980s under former dictator Mobutu Sese Seko, set up Congo's first opposition party in 1982 and opposed Kabila's father Laurent after he seized power in 1997.

He boycotted the first post-war polls in 2006, saying the process was flawed. He spent several years abroad before returning last year to launch his challenge, drawing thousands onto the streets of Kinshasa, where he also is popular.

His UDPS party says he sought treatment in Belgium for an unspecified medical condition but adds he is in good health now.

ETHNIC FAULTLINES

His Kasai bastions of more than five million voters -- a sixth of the electorate -- are home to Congo's richest diamond fields. But their fortunes plummeted as mismanagement and graft pushed the formal mining sector into collapse three years ago.

Mbuji-Mayi is now dilapidated, lacking running water and electricity much of the time. Abandoned tanks, reminders of Congo's bloody past, lie by the side of potholed roads.

He doesn't have personal interests ... We have to have elections so that the old man can bring democracy, said Mako Nkongolo, part of a crowd gathered on a street corner to heap adoration on a man they call Mandela of Congo.

The atmosphere here has deteriorated as local civic rights groups have accused pro-Kabila agents of buying up voter cards at $2 each in a bid to limit the damage in a region that voted heavily against him in 2006.

Alphonse Ngoyi Kasanji, the governor of eastern Kasai and a leading member of Kabila's PPRD party, denied those charges and accused the rival camp of fomenting trouble.

They're preparing their supporters for an uprising, they're probably going to be defeated ... We must take preventative measures, he said without elaborating.

The presidential bid of Tshisekedi, who like most Kasaians is from the Luba people, also risks exposing ethnic faultlines.

That could play out most clearly in the torrid local politics of neighbouring Katanga, the copper-rich province which is still home to thousands of Kasaians despite a series of pogroms and mass expulsions in the 1990s.

Gabriel Kyungu wa Kumwanza, governor at the time and now head of the provincial assembly, was quoted by New York-based Human Rights Watch as saying this year there are too many mosquitoes in the room, now is the time to apply insecticide.

While he denied to Reuters making that comment, he is firm in his view that Katanga should be ruled by its local ethnic groups. Clashes last weekend between his UNAFEC party and supporters of Tshisekedi's UDPS, already left 15 injured.

In the local capital Lubumbashi, crowds gather outside the local UDPS party offices, where supporters with tears in their eyes planted wet kisses on posters of their candidate.

In a sign of the passions to be stirred when voting takes place in two weeks, Tshisekedi supporter Philomene Bilonda, her body shaking with emotion, vowed to sleep at the local polling station to guard against irregularities.

We're going to count the votes, we will die to protect our right to witness the vote, she said.

(Editing by David Lewis, Mark John and Michael Roddy)