Police in Kosovo have made more than 140 arrests after clashing with protesters who tried to stop traffic from Serbia entering the territory.
Around 50 people - most of them police officers - were injured in the violence at two border crossings.
Protesters say Serbia wants to obstruct development in Kosovo, which declared independence from Serbia in 2008.
Serbia doesn't recognise the independence of the mainly ethnic-Albanian territory.
Saturday's demonstrations were organised by Self-Determination, a Kosovo party opposed to contacts with Serbia.
The group, led by hardline opposition leader Albin Kurti, had said it wanted to temporarily block Serb products from entering the territory.
"It's not right to block us and not Serbia," Mr Kurti told demonstrators before police intervened.
"Serbia is the enemy of the Kosovo state and our motto is 'Serbia will not pass'."
Police said more than 30 officers were injured after protesters hurled rocks and metal bars at them in the town of Podujevo, and at the nearby border crossings of Merdare and Dheu i Bardhe.
Police responded with tear gas and water cannon and a Reuters reporter saw several injured protesters in the ensuing clashes.
Police said those arrested included five Kosovo MPs from the Self-Determination party.
Several hours after the violence, both border crossings were open but riot police were still out in force.
The Kosovan government issued a statement after the protest, urging the leaders of Self Determination "to restrain from the use of violence and attempts to block the freedom of movement guaranteed by the constitution".
Ethnic Albanians make up about 90% of Kosovo's population.
The Serbian minority lives in separate areas watched over by Nato peacekeepers.