"It was a provocation by a group of policemen who started beating people," Kakha Kukava, an opposition leader, said.
Sophio Jajanashvili, a spokeswoman for the opposition Georgia's Way party, said: "At least seven protesters were beaten by the police, they were hospitalised with broken heads and legs."
Television pictures showed several wounded protesters, one with blood streaming from his head.
Earlier, hundreds of demonstrators had marched through the centre of Tbilisi
and rallied outside parliament.
The clashes have sparked fears that protests, which have been continuing in the capital for seven weeks, are spiralling into widespread unrest.
Threats by activists to block the main highway and railway line have also deepened fears that violent confrontations could break out in the former Soviet republic.
The opposition are demanding the resignation of Saakashvili over his handling of last year's conflict with Russia and claims his rule is becoming increasingly autocratic.
Critics say the 41-year-old leader has monopolised power since the 2003 Rose Revolution that swept him to power.
Demonstrators are planning to rally again on Friday in a bid to build momentum against the leader.
But Saakashvili has resisted calls to resign, and has pointed to the largely peaceful protests in the capital as a sign that democracy is maturing in Georgia.