Hundreds of protestors, most of them former political prisoners and refugees from Abkhazia, gathered near Hotel Sheraton, the primaries headquarters, but policemen blocked the way to the hotel with a metal fence.
The moment the United National Movement leaders walked out of the hotel they were pelted with eggs, which was followed by a scuffle between protesters and policemen.
Amnestied political prisoner Merab Gorgadze was detained, which incited a fresh wave of discontent.
Another protestor, Merab Gogoberidze, was also detained after he attempted to break through the police cordon.
"We want to show Georgia and the rest of the world that there is no place for these outright cynics in our society. They must be jailed," a demonstrator told the press.
A United National Movement leader, ex-speaker of the Georgian parliament Mikheil Machavariani blamed the authorities for the unrest.
"Unfortunately, what happened in Zugdidi yesterday continues in Batumi today," he said.
"It's a shame if the authorities knowingly organize all this. But it is even worse if they cannot stop such actions," he added.
Reports said earlier that bottles and stones were hurled at the United National Movement leaders when they arrived in Zugdidi for the primaries.
Twelve protestors were detained on petty hooliganism and breach of peace counts, and they were each fined about $60.
Voice of Russia, Interfax