Demonstrators threw stones and bottles and damaged vehicles during the protest in the southwestern town of Struga.
The plan is part of the Ohrid agreement of 2001 that ended seven months of clashes between Macedonian security forces and ethnic Albanian factions calling for greater rights. Of Macedonia's 2 million people, some 25 percent are Albanian.
The plan is due to be debated by parliament on 26 July.
If adopted, the capital, Skopje, will become a bilingual city, with all road signs and official documents issued in both Macedonian and Albanian.
Opposition leaders say the plan will divide the country along ethnic lines.
The plan is part of the Ohrid agreement of 2001 that ended seven months of clashes between Macedonian security forces and ethnic Albanian factions calling for greater rights. Of Macedonia's 2 million people, some 25 percent are Albanian.
The plan is due to be debated by parliament on 26 July.
If adopted, the capital, Skopje, will become a bilingual city, with all road signs and official documents issued in both Macedonian and Albanian.
Opposition leaders say the plan will divide the country along ethnic lines.