After announcing the arrests, police showed journalists around 20 minutes of video they said was secretly filmed of the suspects buying weapons, which included automatic rifles.
"This operation was not aimed against any political party or the demonstration," Shota Utiashvili, the interior ministry spokesman, said.
"We are not going to restrict demonstrations. What we are going to do is make sure that nobody gets shot and that we don't end up in large-scale violence."
President under pressure
The opposition has stepped up pressure on Mikheil Saakashvili, the president, since Georgia's defeat by Russia in a five-day war last August, and plans a series of protests beginning on April 9 to demand his resignation.
of Georgia has resorted to illegal and deplorable methods to fight its political opponents"
Democratic Movement-United Georgia statement
The opposition accused police of planting the weapons at the homes of some of the activists so they could be arrested.
"The government of Georgia has resorted to illegal and deplorable methods to fight its political opponents," the party said in a statement.
"The police regime has launched a full-scale attack against the Democratic Movement-United Georgia."
In recent weeks, Georgian officials have warned that Moscow could try to overthrow the government from within, claims that the opposition says are part of a smear campaign to discredit it.
Gela Bezhuashvili, the head of the Georgian intelligence services, told parliament on Friday that Russia was planning "to remove the Georgian authorities through internal disorder and destabilisation".
Burdzhanadze is a former ally of Saakashvili, but then split with the president in early 2008, criticising his record on democracy.
Her defection came months after police used tear gas and water cannon to crush anti-government protests in the capital, Tbilisi, in November 2007 and shut down an opposition television station.
The government at the time said the opposition was trying to stage a coup.