Thousands of people have gathered in Georgia's capital, Tbilisi, to protest against the government in the largest such rally in the country since 2003.
Many in the crowd were supporters of Mr Okruashvili
The marchers accused President Mikhail Saakashvili of corruption and praised his former ally, Irakly Okruashvili, who was arrested on Thursday.
Mr Okruashvili was arrested two days after he said Mr Saakashvili had plotted to kill a top businessman.
The government says Mr Okruashvili's charge is "false and baseless".
The president is in New York visiting the UN General Assembly and has not personally responded to Mr Okruashvili's allegations.
Massive street protests propelled Mr Saakashvili to power in 2003.
Mr Okruashvili was his defence minister until his dismissal last year. He now leads an opposition party.
His arrest on corruption charges late on Thursday has caused a storm in a country still recovering from years of political turmoil, the BBC's Matthew Collin in Tbilisi says.
At least 5,000 protesters are estimated to have gathered on Friday at Rustaveli Avenue in Tbilisi, outside the parliament building.
Mikhail Saakashvili accuses Russia of meddling in Georgia
They accused President Saakashvili of corruption and authoritarianism and demanded he stand down.
"The main question today is: them or us," Goga Khaindrava, a former cabinet member who joined the opposition, told the crowd.
"We came here to stay, they will have to go," he said.
Security forces armed with riot gear and water cannons formed a cordon around the protesters.
The rally is believed to be the largest street protest Georgia has seen since the so-called "Rose Revolution" that brought Mr Saakashvili to power.
The American-educated president sought to strengthen Georgia's ties with the West and Nato - a policy that has irked Moscow.
Earlier this week, Mr Saakashvili made a speech at the UN Security Council, accusing Russia of fomenting unrest in the break-away region of Abkhazia.
Georgian troops regularly clash with separatist fighters in the break-away regions of Abkhazia and South Ossetia.
During his tenure as defence minister, Mr Okruashvili was known for advocating aggressive military action against the separatists.