Authorities say 10 people were injured in a series of overnight attacks in Macedonia's capital, Skopje.
The first attack occurred in a majority Macedonian suburb when a group of masked, hooded youths armed with baseball bats got on a public bus and injured five passengers.
Another group of youths pelted a bus with stones, injuring a driver and two passengers in a majority Albanian suburb.
Two people were slightly injured in a third incident.
Interior Minister Gordana Jankulovska condemned the violence.
Relations between the Macedonian majority and the ethnic Albanian minority remain scarred by an armed conflict in 2001 between Macedonian security forces and ethnic Albanian rebels.
Ethnic Albanians make up some 25 percent of Macedonia's population of 2 million.
The U.S. Embassy in Skopje condemned the violence.
With AFP and local reports