PRISTINA, Kosovo (AP) — Tension flared in a familiar section of the Balkans as thousands of people marched Saturday in Kosovo's capital against a possible territory swap with former war foe Serbia, while 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 Saturday. 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 about the incident.

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 normalize relations as a precondition to EU membership.

Thaci has mentioned a "border correction" being part of the discussions. Serbian President Aleksandar Vucic has suggested an exchange of territories could help end the dispute.

One idea that has been floated by politicians in both countries involves exchanging predominantly ethnic Albanian Presevo Valley in Serbia with Kosovo's Serb-populated north.

However, the idea has faced opposition from Germany and other EU nations, which have said 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 Thaci a collaborator with Serbia and called for fresh elections.

"Such a grandiose protest is our response to the deals from Thaci and Vucic," Kurti said.

The border correction Thaci mentioned would bring southern Serbia's Albanian-dominated Presevo Valley into Kosovo. However, the Kosovar leader has rejected both border revisions based on ethnicity and a possible land trade.

He has not clarified how Serbia could be persuaded to give away the Presevo Valley without something in exchange.

In a move that temporarily redirected attention from the protest, Thaci on Saturday visited a lake in northern Kosovo that Serbian leader Vucic visited two weeks ago. Kosovo police said a security unit was dispatched to the area to prepare for Thaci's stop.

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 both Pristina and Belgrade have 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," Thaci said Saturday. 

Semini reported from Tirana, Albania. Jovana Gec contributed from Belgrade, Serbia.

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