Hours before his dramatic arrest, Fonseka told reporters that he was willing to face any international probe into alleged war crimes committed by Sri Lankan troops in last year's final stages of conflict with Tamil Tiger rebels.

Fonseka has been moved to an undisclosed military establishment ahead of disciplinary proceedings, a military official said.

"He will face a court martial even though he is not a serving officer," said the official, who declined to be named. "Military law applies up to six months from the date of retirement of any officer."

Fonseka, 59, the only four-star general in the army, quit in November after falling out with his commander-in-chief, President Mahinda Rajapakse.

In a brief statement posted on its website, the defence ministry said Fonseka would be charged with "certain fraudulent acts and other military offences".

Just prior to his arrest, Fonseka had said: "I am not prepared to protect anyone, if they have committed war crimes."

The government has resisted international calls for an investigation amid charges that a senior defence official ordered the killing of surrendering rebel leaders. Related article: Sri Lanka under fire over opponent's arrest

The United Nations says 7,000 civilians died during the final stages of fighting.

Fonseka's arrest caps a dramatic fall from grace for a man who, eight months ago, was regarded by many as a national hero.

As the battlefield architect of the military victory over the Tigers last May, Fonseka had seemingly secured his legacy as the commander who finally crushed their 37-year campaign for an independent Tamil homeland.

But his bid to translate that success into political power proved his undoing when Rajapakse easily saw off his ballot-box challenge on January 26.

The two men had fallen out soon after the Tigers' defeat, and both sought to take credit for the victory in what became an acrimonious battle for the presidency.

Fonseka was holding meetings at the offices of the opposition parties that had supported his candidacy when military police came for him on Monday night.

"They grabbed him and virtually carried him away," a spokesman for the People's Liberation Front told AFP. "There must have been over 100 soldiers."

Fonseka's campaign manager, Senaka de Silva, was also detained, witnesses said.

A day earlier a private newspaper said Fonseka could be court-martialled on charges of plotting a coup.

Earlier this month, Rajapakse sacked a dozen senior military officers who the defence ministry said were a "direct threat" to national security.

Fonseka had vowed to challenge the result of the January poll in the Supreme Court, and claimed the government had arrested more than 50 people from his office to prevent evidence being collected to back his claim of vote fraud.

He had also been keen to contest parliamentary elections, which the government is expected to call within days.

His detention prompted concern from the United States, which said it could worsen divisions as the island recovers from war.

"There is a tremendous need for the government of Sri Lanka to work to overcome the fissures that exist within its society," State Department spokesman Philip Crowley told AFP.

"It has to be very cautious that any actions it takes are designed to heal the split within Sri Lankan society, not to exacerbate it," he said.

Another official in Washington told AFP that US diplomats had been working behind the scenes to encourage caution from Rajapakse.

Fonseka holds a "Green Card" giving him residency rights in the United States.