By Patrick Nduwimana

BUJUMBURA, April 25 (Reuters) - President Pierre Nkurunziza is widely expected to be chosen to run for a third five-year term at a gathering of Burundi's ruling party on Saturday, a move critics say would be unconstitutional and may trigger unrest.

Opposition groups have vowed protests if Nkurunziza stands again as this, they say, would undermine a peace deal that kept the poor east African nation calm for a decade since the end of an ethnically-fuelled civil war in 2005.

Two months ahead of the presidential election, close to a thousand members of the party representing the former Hutu rebel movement, including Nkurunziza, were attending the meeting in its complex in the capital Bujumbura.

"(The) CNDD-FDD is preparing to win the election in a democratic way, and there is no doubt we will win," party chairman Pascal Nyabenda said in his opening speech.

Security outside the complex and on all streets of Bujumbura has been tightened to deal with any eventual protest that could erupt in case the party nominates Nkurunziza.

"We call upon participants to that meeting not to elect Nkurunziza because that will be a violation of the constitution and the Arusha peace agreement," prominent civil society activist Pierre Claver Mbonimpa said.

"Once Nkurunziza is declared candidate, we won't wait. The next day (Sunday), over 300 civil organisations engaged in a campaign against a third term for Nkurunziza will descend in the streets," he told Reuters.

Supporters of the president have argued it is permissible for him to run in the June 26 election as his first term should not count because he was chosen by parliament rather than having been voted into office.

Some opponents staged street protests this month before the announcement. On April 17, demonstrators scuffled with police. Any clampdown on rallies could stoke tensions in a nation which has suffered from decades of ethnic violence.

(Additional reporting by Edmund Blair in Nairobi; Writing by George Obulutsa; Editing by Mark Heinrich)

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