Thousands hit the streets of Tbilisi to demand the release from prison of Georgia's ex-president Mikheil Saakashvili.
Saakashvili, president of Georgia from 2004 to 2013, was arrested and imprisoned on October 1 when he returned from exile in Ukraine.
He said he risked the return in order to try to galvanise the opposition ahead of local elections.
The 53-year-old pro-Western reformer and founder of Georgia’s main opposition force was convicted in absentia on charges of abuse of office and sentenced to six years in prison in 2018. He has denied any wrongdoing.
Saakashvili announced a hunger strike hours after his arrest and his doctor said this week that his condition was deteriorating.
His supporters want the country "to return to its pro-Western path".
"It is not only just to save Mikheil Saakashvili as a president. First of all, we have to save our future," said Nika Melia, chairman of the United National Movement.
"We do not want to have Lukashenko's regime in Georgia. We do not want to be the state which is very far from being a democracy. Today's evening is very promising."
Saakashvili and his supporters say the charges against him are politically motivated and refer to him as a political prisoner.
The rally of some 30,000 people in Tbilisi's Freedom Square on Thursday underlined tensions ahead of mayoral runoff elections in Georgia's six largest cities scheduled for October 30.
Opposition leaders believe that success in the run-off rounds "will dramatically change the picture in the country".
Ahead of the demonstration, Prime Minister Irakli Garibashvili said Saakashvili was attempting to "stir up unrest and destabilise the country".