The Coalition for Change party published video on X showing Nika Gvaramia, the party's leader, being carried by the arms and legs by several men down some steps.
The party said that Gvaramia, a 48-year-old lawyer and media manager-turned politician, had been "thrown into a detention car as he was physically assaulted and unconscious".
The police did not immediately respond to a Reuters request for comment and there was no immediate response to the assertion by the authorities, who have faced six nights of protests against a government decision to suspend talks on the country joining the European Union.
Reuters could not independently verify whether Gvaramia had been beaten or not but he did not appear to be moving as he was carried down the steps in the video released by his party.
Asked at a press conference about claims authorities were repressing the opposition, Prime Minister Irakli Kobakhidze said: "I would not call it repression, it is more prevention."
He alleged that opposition forces had been supplying protesters with fireworks, which they have hurled at police during demonstrations.
"People were systematically supplied with pyrotechnics and other means by the relevant political forces," Kobakhidze said.
The government's decision to suspend EU talks has plunged the South Caucasus country of 3.7 million people into political crisis and the authorities claim to have thwarted an attempted revolution.
Critics accuse the government of turning its back on the EU and steering an increasingly authoritarian and pro-Russian course, which the ruling party denies.
Salome Zourabichvili, the country's pro-EU president who has become a voice of the protest movement, wrote on X on Wednesday:
"My urgent call to our partners and those who want to prevent (the) crisis to go deeper..., it is time to put strong pressure on a ruling party that is driving the country over the cliff! Do not be late … !"
Zourabichvili - who plays a largely ceremonial role - has declared that she will not step down even after her six-year term ends later this month, aiming to stay in the job to lead the opposition demand for a new parliamentary election.
Kobakhidze said on Tuesday that the government is willing to open EU accession talks if the bloc ends its "blackmail".
"I want to remind European bureaucrats and politicians, including those who are artificially hindering our country's European integration, to bring negotiations to the table, and we will sign immediately, on the same day, at that very moment," he said.
A spokeswoman for Coalition for Change said on X that several other party members had been detained alongside Gvaramia.
A spokesperson for the United National Movement, another opposition party, told Reuters that police on Wednesday had also raided its Tbilisi offices and arrested five of its members.
Separately, the Interpress news agency said that two members of another opposition party, Strong Georgia, had been detained by the police.
A leader of Georgian youth protest movement Dafioni was also arrested, Georgian media reported.
Kobakhidze, the prime minister, has repeatedly praised the police for their response to the protests.
Georgia's public ombudsman, a former opposition politician, accused the police on Tuesday of harshly mistreating people detained during demonstrations, saying their treatment amounted to torture.
with AP
