Interior Minister Vano Merabishvili said that the detainees were all supporters of fugitive opposition leader Igor Giorgadze and had been arrested in raids across the country.
"They will be charged under article 315 of the Georgian criminal code -- plotting against the state and overthrowing the government," Mr Merabishvili said, without giving details of the alleged plot.
Georgian state and private television showed footage of black-clad security forces leading several opposition figures into waiting cars.
Footage was also aired of what was allegedly a weapons stash discovered in the basement of a house linked to the opposition.
Security increased
Meanwhile, armoured personnel carriers were deployed on roads entering the capital Tbilisi.
Interviewed by Russia's state-run Channel One television, Giorgadze said the arrests were part of a campaign of repression ahead of municipal elections in October. "This is the begining of the end of Saakashvili (President Mikhail Saakashvili )," he predicted.
In a separate interview with Russia's Echo Moscow radio, he also warned that "if the authorities raise their stick..., then we will answer with a stick."
Relatives and lawyers of those detained said that between 15 and 30 people had been arrested, all of them close to Giorgadze.
These included Conservative Monarchist Party leader Temur Zhorzholiani, two regional representatives of the Justice Party and the leader of the Anti-Soros movement, named after US philanthropist and Saakashvili supporter George Soros.
Zhorzholiani's lawyer, Gela Nikolaishvili, said that his client denied treason.
"Zhorzholiani took part in a meeting which allegedly occurred on May 24 with the participation of activists from the Anti-Soros movement and Justice Party where they allegedly plotted a coup masterminded by Igor Giorgadze. Zhorzholiani says the allegations are false," the lawyer said.
Assassination accusations
For a decade, Giorgadze has been a fugitive from Georgia where he is sought on charges of attempting to assassinate former president Eduard Shevardnadze in 1995.
He is also accused of preparing a coup against the government in Georgia, an impoverished but strategic country of fewer than five million people.
He lives in hiding but held a high-profile press conference in May in
Moscow and has been offered political asylum by Russia, prompting an angry reaction from the US-backed government of President Saakashvili in Tbilisi.