Riot police used batons on protesters trying to enter a police compound where three people were being held over the alleged beating of a local journalist.
The clashes were the first major unrest since anti-government demonstrations began in early April.
They come a day after the authorities said they had thwarted an army mutiny at a base outside the capital.
Later in the evening, opposition leaders and supporters gathered outside parliament for a rally, as they have daily since 9 April.
The BBC's Tom Esselemont in Tbilisi says attendance at the rallies has dwindled but tensions in the city appear to be rising.
The protesters are calling for the resignation of President Mikhail Saakashvili over his leadership record and his handling of Georgia's war with Russia last summer.
'Small scuffle'
The latest clashes broke out after dark in a suburb of Tbilisi after demonstrators, including opposition party leaders, surrounded the police station.
They were demanding the release of three opposition activists arrested over the alleged beating of a reporter for a public broadcaster on Tuesday.
Interior Ministry spokesman Shota Utiashvili said riot police used batons to force back protesters trying to break into the building and there had been "a small scuffle".
Mr Utiashvili denied accusations from protesters that police had used "excessive force", including firing rubber bullets into the crowds.
He said saying the police had acted "with the utmost restraint".
Television footage showed several injured protesters, including some with blood on their faces.
The opposition said dozens of their supporters were injured, while Deputy Interior Minister Eka Zguladze put the number at 22, with six police officers also wounded.
Later, several thousands protesters converged on parliament for further rallies.
"This president has dug his own grave," said opposition leader Nino Burjanadze.
"We'll be standing and fighting till the very end."
Distraction
On Tuesday, Mr Saakashvili claimed his government had put down a brief mutiny among a tank battalion.
Tbilisi had said it was part of a Russia-linked coup attempt to kill Mr Saakashvili.
But opposition parties said the alleged mutiny was a deliberate attempt by the government to distract attention from the new phase of anti-government protests.
The latest unrest comes as Georgia hosts a series of Nato training exercises amid angry condemnation from Russia.
They are taking place close to areas where Russian troops are stationed in Georgia's breakaway region of South Ossetia.
Russia's President Dmitry Medvedev said the drills, involving more than 1,000 soldiers from 18 countries, were "an overt provocation".
