Georgia's opposition parties have agreed on a single candidate to challenge President Mikhail Saakashvili in snap polls to be held in January.
Mr Gachechiladze (far left) is a known government critic
Levan Gachechiladze, an MP, was at the forefront of last week's rallies calling for Mr Saakashvili to quit.
The pro-Western president called the early election following protests on the streets of Tbilisi which were eventually crushed by police.
The crisis led Mr Saakashvili to declare a 15-day nationwide emergency.
The opposition named Mr Gachechiladze, 43, as their candidate at a news conference in Tbilisi on Monday.
Mr Saakashvili says he wants a renewed mandate
It said former Foreign Minister Salome Zurabishvili would run on the ticket as Mr Gachechiladze's candidate for prime minister.
"It won't be an ordinary election. It will be against violence, it will be against injustice," Mr Gachechiladze said.
It is the first time that Georgia's fractured opposition has united to challenge Mr Saakashvili since he was swept to power in a peaceful "Rose Revolution" in 2003.
Early elections were one of the main demands of protesters.
The opposition accuses Mr Saakashvili of corruption and authoritarianism - charges he denies.
Some 500 people were injured during last week's clashes
The president announced the early elections after widespread shock in Georgia at the ferocity of the police crackdown on protesters in central Tbilisi on Wednesday.
Police used tear gas, water cannon and rubber bullets to disperse several thousand protesters. Some 500 people were injured.
There were also strong expressions of concern from the president's allies in the West over the declaration of a state of emergency.