The raid came a day after Georgian police stopped a Russian military convoy in South Ossetia, a move that sparked a row with Moscow.
Armed South Ossetian separatists this morning entered a mostly ethnic Georgian village of Vanati and disarmed and detained some 30 to 40 Georgian members of a peacekeeping force stationed there.
Shortly after, in a separate incident, authorities in Tbilisi said two Georgian soldiers were injured in a clash with South Ossetian forces elsewhere in the region.
The violence has raised tensions in the province, which Tbilisi wants returned to central government control after a decade of self-declared independence.
Georgian Prime Minister Zurab Zhvania called the raid "an unprecedented, insolent provocation."
Zhvania promised immediate action to free the peacekeepers, although he said the conflict must be resolved peacefully.
"We can say that we have the situation under control," Zhvania said. "And I want to stress, especially now, when the situation is so tense and emotions are very high, that the policy of President [Mikheil] Saakashvili and our government is for peaceful dialogue with [South] Ossetia."
South Ossetia's self-declared government defended the raid, saying the detained men are not peacekeepers at all, but troublemakers.
But Georgian State Security Minister Vano Merabishvili blamed South Ossetian President Eduard Kokoity for escalating tensions in the province.
"According to our information, Kokoity is ready to start a war because he knows very well that this is the only chance for him to stay in power," Merabishvili said.
"The policy of President [Mikheil] Saakashvili and our government is for peaceful dialogue with [South] Ossetia." -- Georgian Prime Minister
Today's violence is the latest in a string of incidents that have raised fears that South Ossetia could erupt into armed conflict.
The tiny mountainous northern province, on the border with Russia, is the scene of a tense three-way standoff between Georgia, South Ossetia's self-proclaimed breakaway regime, and Russian peacekeepers, who many in Georgia suspect of siding with the separatists.
Today's incidents come a day after Georgia intercepted a Russian convoy carrying arms in South Ossetia, a move that raised tensions between Tbilisi and Moscow.
Georgia accused Russia of supplying the separatist South Ossetian leadership. But Russia said the equipment was destined for Russian peacekeepers and had been agreed ahead of time.
A spokeswoman for South Ossetian President Kokoity, Irina Gogloeva, described the incident as an "armed incursion" into South Ossetian territory and a provocation that could lead to war.
Today, Russia's Foreign Ministry said Moscow is "extremely worried" about the situation in the province.
"Moscow calls on the parties in the conflict to exercise maximum restraint and prevent actions that would violate legal norms and cause further complications in the situation," Foreign Ministry spokesman Aleksandr Yakovenko said.
(RFE/RL's Georgian and North Caucasus services/ wire reports)