"If just one hair falls from the head of a member of my family or my friends, I will spend the rest of my life revenging Saakashvili wherever he may be," Irakly Okruashvili said in an interview with the Imedi TV channel from Munich.
He said he knew that after the charges he had leveled against the Georgian authorities, he would come under pressure, but did not expect such intense pressure.
"I could not even imagine that my friends could suffer as a result of my remarks," he said, adding he had to stay abroad as a political emigre, but would soon return to Georgia.
Georgian opposition groups have been rallying in central Tbilisi since Friday, demanding that the government bring parliamentary elections forward to April, threatening to continue protests, which saw around 50,000 supporters gather in front of government buildings and was the country's worst unrest since the 2003 "rose revolution" that swept Mikheil Saakashvili to power.
Protesters put up mock gallows in front of the parliamentary building with a rose stem instead of rope with Saakashvili's photograph attached to the end.
Okruashvili was earlier forcibly flown out of the country to Europe to prevent him from taking part in the rally. Officials said he went abroad for medical treatment.
He urged the protesters to show restraint but stand firm, adding "Together, we must pool our efforts and get rid of Saakashvili."
On Tuesday, the Georgian Prosecutor General's Office accused Okruashvili of lying.
"I categorically deny that Okruashvili testified under duress. That is yet another lie from him," said Nikoloz Gvaramia, deputy prosecutor general, adding that a lawyer had been present at all stages of the investigation.
In late September, Irakly Okruashvili, who was dismissed as defense minister in November 2006, accused President Mikheil Saakashvili of ordering the murders of political opponents. He was subsequently arrested and released on October 9 on $6 million bail after retracting the accusations in a televised interview with state prosecutors.
The opposition later claimed that psychotropic drugs had been used against Okruashvili in order to get him to retract his accusations. The claims were denied by the Georgian government.
An opposition spokesman reported previously that Okruashvili had decided to quit politics.
Georgian prosecutors said Okruashvili had asked for permission to leave the country for medical treatment, saying that he had informed them on October 29 that he required "medical treatment outside of Georgia." They added that they had given their consent for this.
Saakashvili, who has pushed for NATO membership for the Caucasus state and has enjoyed Western backing in his ongoing disputes with Russia, has recently faced growing protests at home. Protests increased after Saakashvili ordered the arrest of an ex-defense minister, Irakly Okruashvili, who accused him of corruption and plotting to kill Patarkatsishvili.