* Rwasa commanded FNL rebel group during civil war
* Political crisis worst since war ended in 2005
* Says Burundians fed up with graft, slow growth
By Richard Lough
NAIROBI, Feb 28 (Reuters) - Burundi's worst political crisis since the end of its 12-year civil war risks unleashing a new wave of unrest before next year's presidential election, the main opposition leader said on Thursday.
Agathon Rwasa, the last rebel commander to lay down arms in 2009, accused President Pierre Nkurunziza of seeking to rewrite the constitution for his party's own gain and of behaving increasingly like a dictator.
The turmoil in the east African country centres on a row between Nkurunziza's Hutu-led CNDD-FDD party and its junior coalition partner, Uprona, over constitutional amendments proposed by the president that could allow him a third term.
"If it continues, it could lead to a situation where people say enough is enough," Rwasa told Reuters by telephone from his home in Burundi's capital, Bujumbura.
"Everybody is anxious about what is going to happen. Some people could go crazy and come back to (using) weapons."
The political standoff has raised the spectre of more unrest in a region grappling with violence in eastern Democratic Republic of Congo, South Sudan and the Central African Republic.
Rwasa commanded the National Liberation Forces (FNL) during the 1993-2005 civil war that was triggered by the assassination of the country's first democratically elected president, an ethnic Hutu, after decades of post-colonial Tutsi rule.
The FNL was among several Hutu rebel groups to rise up to fight the Tutsi-led military government. The biggest was Nkurunziza's CNDD-FDD, which in the early 2000s negotiated an end to rebellion in return for government and military jobs.
Nkurunziza was elected president by lawmakers in 2005. His army battled the more hardline FNL until it laid down arms in 2009. Rwasa, accused by the government of committing some of the worst atrocities during the conflict, is widely viewed as posing the biggest threat to Nkurunziza's leadership at the ballot box.
Nkurunziza has not said he will run again and his supporters say the president has stuck by the rules of the constitution in keeping a careful balance and fully representative government.
"GOODBYE TO WEAPONS"
Rwasa said it was false to depict the current crisis as an ethnically motivated spat between the CNDD-FDD and predominantly Tutsi Uprona, citing widespread discontent among the Hutu community at rampant corruption and sluggish growth.
"The problem is bad governance," said Rwasa.
The government does not consider Rwasa the legitimate leader of the FNL, a party that has faced infighting. The opposition say rifts were stoked by the government to dilute the challenge.
Asked if the crisis might persuade him to re-arm and return to the bush, the former fighter said: "No, I said goodbye to using weapons to claim my rights."
The crisis blew up after Nkurunziza sacked his vice president this month, prompting Uprona ministers to walk out of the coalition in protest.
Nkurunziza appointed a new deputy and three more ministers from Uprona, as required by an ethnic power-sharing deal in the constitution, but the party rejected their nominations, saying they did not represent the mainstream.
Nkurunziza wanted to split Uprona and its support base before next year's election, as it has done with Rwasa's FNL and other opposition parties, analysts said.
"What is happening to Uprona has happened before to us. And they are doing their best to prevent any kind of unification of our party," Rwasa said.
Rwasa, who united with other opposition leaders to boycott the 2010 presidential election, said he would run for president in 2015 if chosen by FNL members, but said the political chaos undermined his hopes for a free and fair vote.
"I am not very optimistic," he said.
Rwasa, who disappeared into hiding after the 2010 poll saying he feared for his life, only emerging last August, said he and colleagues were frequently harassed by state agents and the party was unable to operate freely.
Rights groups have reported scores of political killings, intimidation of the opposition and a crackdown on media freedoms since Nkurunziza's re-election in 2010.
"I must be very vigilant, my movements are very limited," Rwasa said. (Additional reporting by Patrick Nduwimana; Editing by Edmund Blair and Janet Lawrence)