An ally-turned-opponent of Georgian President Mikheil Saakashvili, Irakli Okruashvili, has gone on trial, charged with setting up an illegal armed group.
The charges were brought last year after opposition protests against Mr Saakashvili's rule.
Okruashvili, 39, was then living in France, which refused to extradite him.
Last month he returned to Georgia, where he was sent to jail to serve a previous 11-year term for extortion, handed down in absentia in 2008.
Political allies said he was determined to clear his name over the extortion conviction.
President Saakashvili's political position was seriously weakened when his United National Movement party lost October's general election, in a shock defeat to the Georgian Dream coalition led by Bidzina Ivanishvili, who is now prime minister.
Okruashvili has repeatedly accused Mr Saakashvili of criminal activity, in particular of being connected to the mysterious deaths of political rivals.
If those allegations are made in court, analysts say the president himself could face questioning, the BBC's Damien McGuinness reports from Tbilisi.
Born in Tskhinvali, the capital of Georgia's breakaway region of South Ossetia, Okruashvili is a lawyer by training with a reputation as a strong nationalist and anti-Russian hawk.
After the pro-Western "Rose Revolution" of 2003, Mr Saakashvili promoted him to three top posts inside a single year: chief prosecutor, interior minister and finally defence minister.
In April 2006, shortly after a controversial Russian ban on imports of Georgian wine on grounds of food hygiene, he famously declared that one could sell "faecal matter in Russia".
After being moved to the ministry of economic development that November, he resigned from the post after barely a week, and his relations with the president deteriorated.
The following year, he set up his own political party and accused Mr Saakashvili of planning the murder of businessman Badri Patarkatsishvili, as well as questioning the mysterious death of Prime Minister Zurab Zhvania.
Shortly afterwards, he was arrested for extortion while in office but was allowed to move abroad while on bail. His trial eventually resumed without him, and he was convicted in absentia in March 2008.
On 28 June last year, Georgian prosecutors charged him with "creating or leading an armed formation".
He had allegedly planned to seize power in Georgia with the help of "200 Russian commandos" - a charge his allies described as "absurd".
The prosecution followed anti-Saakashvili demonstrations in Tbilisi, during which Okruashvili promised to return from exile to take part personally.