Tension flared in a familiar section of the Balkans as thousands of people marched in Kosovo's capital on Saturday against a possible territory swap with former war foe Serbia.
The Serbian government put its troops on alert after special police were deployed to Kosovo's Serb-dominated north.
Serbia reacted after Kosovo's special police moved into an area around the Kosovo side of the strategic Gazivode Lake, Marko Djuric, director of Serbia's Office for Kosovo and Metohija, said.
Kosovar President Hashim Thaci visited the area near Serbia's border on Saturday, a move that temporarily redirected attention away from the large opposition protest in Pristina. A security unit was dispatched to the area for the president's stop, Kosovo police said.
Serbia's Mr Djuric said special troops must not be deployed unannounced to northern Kosovo, where the country's ethnic Serbian minority population is concentrated. Serbian media said Belgrade complained to NATO Secretary General Jens Stoltenberg.
Serbia does not recognize Kosovo's 2008 declaration of independence, but their governments have been in European Union-mediated negotiations for seven years. The two sides have been told they must normalise relations as a precondition to EU membership.
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President Thaci said a "border correction" could be part of the discussions. Some Serbian officials have suggested an exchange of territories could help to end the dispute.
One idea that has been floated by politicians in both countries involves exchanging predominantly ethnic Albanian Presevo Valley in southern Serbia with Kosovo's Serb-populated north.
However, the idea has faced opposition from Germany and other EU nations. They fear a Kosovo-Serbia trade could trigger demands for territory revisions in other parts of the volatile Balkans.
Thousands of supporters of Kosovo's opposition Self-Determination Party marched peacefully through the capital of Pristina on Saturday to protest any potential change of borders. The protesters held national Albanian flags.
Opposition leader Albin Kurti said he considered President Thaci a collaborator with Serbia and called for fresh elections.
"Such a grandiose protest is our response to the deals from Thaci and Vucic," Mr Kurti said.
President Thaci has rejected both border revisions based on ethnicity and a possible land trade.
But he has not clarified how Serbia could be persuaded to give away the Presevo Valley without something in exchange.
Three weeks ago, Serbian leader Aleksandar Vucic visited the lake in northern Kosovo that President Thaci traveled to on Saturday.
Nato-led peacekeepers in Kosovo, a force known as KFOR, called for calm and restraint. They said they would continue monitoring the situation along the Serbia-Kosovo border with ground patrols and helicopters.
President Thaci's office issued a statement acknowledging his visit to a border crossing and the lake.
"During the weekends, the head of state usually goes to Kosovo's beauties," the statement said.
The governments in Pristina and Belgrade said they hope the EU-mediated talks will result in a legally binding agreement.
"Talks [with Serbia] that continue will be on peace and stability," President Thaci said on Saturday.
