President Mikheil Saakashvili announced the elections on Thursday a day after riot police brutally broke up mass protests in the capital calling for his resignation; the president also declared a state of emergency.
During the almost week-long rallies, initially involving tens of thousands of people, the opposition also demanded parliamentary elections in April and changes to the electoral system.
Speaking at a news briefing at the luxury Marriott hotel in Tbilisi, Gachechiladze, 43, reaffirmed his commitment to the opposition's longtime goal of radically reducing the president's powers in the Caucasus state. He said his campaign would also be aimed against violence and political reprisals.
"I will spare no effort to achieve these goals and our victory. But fair elections will be difficult to hold, as violence is raging in our country," he said.
About 600 people sought medical aid after police fired tear gas, water cannons and rubber bullets at protesters. Gachechiladze himself was severely beaten and admitted to hospital. He later called the police crackdown "Saakhashvili's yellow terror", referring to the yellow raincoats worn by police.
Several opposition leaders faced charges of spying for Russia and conspiracy to stage a coup amid the protests, and some were briefly arrested. But Saakashvili said the prosecutions, as well as the police crackdown, were necessary to counter Russia's interference and preserve Georgian statehood.
Both the opposition and the Kremlin denied the allegations, and the opposition underlined their commitment to continue the country's pro-Western course.
Opposition leaders have also complained that the state of emergency and related media restrictions will hamper their campaign.
Opposition members said at the briefing that Salome Zourabishvili, a former foreign minister and an opposition leader, would become prime minister in the event of their victory.
Several other people, including tycoon Badri Patarkatsishvili, have announced plans to run in the early polls. Saakashvili has not made a formal declaration, but has made it clear he will run and expects to win.
Analysts have said the opposition camp lacks a leader to challenge Saakashvili, and it could fail to advance a strong candidate in such a short time, suggesting the president was staking his hopes on that.
The treatment of the political crisis by Saakashvili, who himself was swept to power by street protests in 2003, has triggered sharp criticism in the West.
While backing Saakashvili's policy to integrating the ex-Soviet state with Europe and NATO, the opposition has criticized his increasingly totalitarian tendencies and failure to ensure economic growth.