The protests, which erupted late on March 1 and continued overnight, were initiated by ethnic Macedonians angry at the appointment of ethnic Albanian Talat Xhaferi's as defense minister.
Xhaferi is a former rebel commander in the 2001 conflict that pitted Macedonia's two main ethnic groups against one another.
Ethnic Albanians staged a counterprotest in the capital on March 2.
Both protests turned violent, with Macedonians and Albanians clashing with police.
Police say 18 people were arrested so far, five of whom are minors.
During the second protest, police used tear gas to disperse crowds after protesters demolished a bus station in front of the government building chanting "UCK" -- a reference to the former ethnic Albanian rebel force -- and pelted the police with stones.