Exiled tycoon Badri Patarkatsishvili has said he will run in snap presidential elections called to quell an escalating crisis in Georgia.
Mr Patarkatsishvili - whom authorities accuse of plotting a coup - called the government "completely discredited".
His pledge came after government and opposition leaders met in Tbilisi for the first time since police broke up violent protests there on Wednesday.
The severity of the government response to the protests has shocked onlookers.
Despite President Mikhail Saakashvili's pledge to call a presidential poll on 5 January, a state of emergency remains in force banning public demonstrations and independent news broadcasts.
A high-level US state department official is expected to add his voice to European demands to lift the state of emergency when he arrives in Tbilisi on Saturday.
But President Saakashvili - who accuses Russian agents of destabilising the country, a charge which Moscow denies - has said it will remain in force as long as necessary.
Opposition 'surprised'
Mr Patarkatsishvili is an old foe of the government in Tbilisi.
He finances opposition parties and is the owner of Imedi television, which was taken off the air on Wednesday following the tough police crackdown on thousands of protesters.
"The state of emergency will be lifted... not by somebody's recommendation, but when we find it necessary"
On Saturday, Mr Patarkatsishvili told Reuters news agency he would participate in January's elections, in a statement emailed from his location in exile, thought to be Israel.
""Mr Saakashvili's regime has completely discredited itself in the eyes of the Georgian people who will never again entrust it its destiny," the statement said.
Mr Patarkatsishvili's pledge reportedly surprised a 10-party opposition coalition which is trying to unite around a single candidate to challenge Mr Saakashvili.
"It's the first I have heard of this," opposition leader Tina Khidasheli told Reuters.
On Friday, Georgia's prosecutor general's office said Mr Patarkatsishvili was under criminal investigation for plotting a coup and he was wanted for questioning.
Pressure continues
Opposition parties have hailed the calling of the election on 5 January as a victory, but commentators caution that it presents them with the significant task of mounting a credible challenge to Mr Saakashvili in the space of a few weeks.
No opposition figure has emerged who could seriously challenge him in the presidential elections, says the BBC's Matthew Collin in Tbilisi.
CRISIS TIMELINE
The leader of the opposition Republican party, Levan Berdzenishvili, told news agency AFP that discussions over candidatures would only begin after the state of emergency is lifted.
The emergency decree was passed by parliament on Friday, but opposition lawmakers boycotted the vote.
The EU envoy to the South Caucasus, Peter Semneby, urged the state of emergency to be lifted "as soon as possible" on Saturday.
His calls are expected to be echoed by Matthew Bryza, US deputy assistant secretary of state, when he arrives on Saturday.
But Mr Saakashvili sounded a defiant note in a speech on Saturday, saying the decree would be lifted "when we deem it necessary".
Opposition leaders met a government representative, parliamentary speaker Nino Burdzhanadze, for preparatory talks ahead of full negotiations set to begin on Saturday evening.
In addition to elections, opposition demands include bringing forward parliamentary elections, the reform of election rules, the release of jailed opposition figures and the resignation of the president.
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