A former Georgian defence minister, Irakli Okruashvili, has been arrested on charges of money-laundering, abuse of power and extortion, officials say.
Mr Okruashvili's spokeswoman said the arrest was political retaliation
It comes two days after he alleged that the president had instructed him to kill a prominent businessman - claims dismissed by the government as untrue.
Mr Okruashvili, a former ally of the president, left the government in 2006.
Earlier this week, he launched his own opposition party, accusing President Mikhail Saakashvili of corruption.
Mr Okruashvili was detained at the headquarters of his new party, the Movement for a United Georgia.
His spokeswoman described the arrest as political retaliation and said Mr Okruashvili would deny all charges against him.
In a dramatic return to politics on Tuesday, the former defence minister alleged President Saakashvili had asked him to kill several public figures, including a millionaire businessman who founded one of the country's leading television stations.
He said the plan never materialised.
Mr Okruashvili also alleged that Mr Saakashvili's high-profile campaign against corruption was a sham.
The BBC's Matthew Collin in Tbilisi says this has caused a huge scandal in a country that is still struggling to recover from years of turmoil and civil war.
Mr Saakashvili has not responded personally so far, but senior figures in the Georgian government described the allegations as "baseless and untrue".
Mr Okruashvili's arrest comes after a series of officials who are believed to support the former defence minister were also detained on corruption charges.