Thousands marched in Tbilisi demanding the release of jailed opposition leader Mikheil Saakashvili.
Waving Georgia's five-cross flag and holding banners that read "Free Saakashvili!" protesters marched through the Georgian capital before gathering outside parliament for the rally timed to coincide with the politician's 54th birthday.
Protesters vowed to start a "mass hunger strike" until he is released.
Saakashvili's arrest triggered a political crisis stemming from parliamentary polls last year, which the opposition has denounced as fraudulent. It also spurred the largest anti-government protests in a decade.
"Today, we are launching a mass hunger strike that will not end until Mikheil Saakashvili is released from captivity," Nika Melia, the chairman of Saakashvili's United National Movement (UNM), said to the crowds.
It is unclear how many people intend to participate in the hunger strike outside the UNM headquarters.
"This is a non-violent protest, a tough move, we have no other choice but to put pressure on the regime so that it loosens its grip on the Georgian state which it has captured," Melia added.
'We need freedom here and now'
The announcement comes as Georgian President Salome Zurabishvili aims to facilitate a “national reconciliation” to overcome political polarisation and division among the public.
Melia welcomed the Zourabishvili-initiated process of national reconciliation and vowed that the opposition would do its best to play a positive role in this process.
In a message to supporters that was read out at the rally by Saakashvili's mother, Giuli Alasania, the former leader called for national unity and peaceful mass protests to pressure authorities to hold snap parliamentary polls.
He said Georgia's "long-time dream and historic aspiration of European integration is under threat".
"We are in vital need of free media, impartial judiciary, fair elections. We need freedom here and now, and for good."
"Changing the current regime is an essential pre-condition for the fulfilment of our Western aspirations," Saakashvili went on, referring to the ruling Georgian Dream party founded by the powerful oligarch Bidzina Ivanishvili.
'We will not stop until Saakashvili is free'
Georgia's richest man, who made his fortune in Russia, Ivanishvili is widely believed to be the top decision-maker in the country, despite having no official political role.
One of the demonstrators, 47-year-old architect Giorgi Darsavelidze, claimed that "Ivanishvili's regime will crumble under popular pressure".
"We will not stop until Saakashvili is free, until Georgia is back to its European path," he expressed.
On Saturday, an independent council of doctors who examined Saakashvili in custody said he had developed serious neurological diseases "as a result of torture, ill-treatment, inadequate medical care, and a prolonged hunger strike".
Saakashvili refused food for 50 days to protest his jailing for abuse of office, a conviction he has denounced as politically motivated.
The pro-Western reformer called off his hunger strike after he was placed — in a critical condition — in a military hospital in Georgia's eastern city of Gori.
Georgia's president from 2004 to 2013, Saakashvili was arrested on October 1 this year, shortly after he secretly returned to Georgia from exile in Ukraine.
Human rights group Amnesty International has branded Saakashvili's treatment "not just selective justice but apparent political revenge".
The US State Department has urged Georgia's government "to treat Saakashvili fairly and with dignity".
Several other rights groups have accused the Georgian government of using criminal prosecutions to punish political opponents and critical media.
Prime Minister Irakli Garibashvili sparked an uproar recently when he said the government had been forced to arrest Saakashvili because he refused to quit politics.
The former president is now also facing additional charges, including illegal seizure of property, embezzlement, illegal rally dispersal, and illegal border crossing.