PRISTINA, Kosovo – Thousands of people protested Saturday in front of the capital of Kosovo in a known area of ​​protesters against a possible exchange of territory with the former enemy of the Serbian war, while the Serbian government was putting its troops in state alert after the deployment of special police forces against Kosovo Serbs. North dominated.

Serbia reacted after Kosovo's special police moved into an area around Gazivode's strategic lake on the Kosovo side, said Marko Djuric, director of the Serbian office for Kosovo and Metohija.

Kosovar President Hashim Thaci visited Saturday in the region near the Serbian border, which temporarily diverted attention from the big opposition demonstration in Pristina. A security unit has been sent to the region for the arrest of the president, the Kosovo police said.

Djuric of Serbia said special troops should not be deployed without warning in northern Kosovo, where the Serbian minority population is concentrated. Serbian media said Belgrade complained to NATO Secretary General Jens Stoltenberg.

Serbia does not recognize the 2008 Kosovo Declaration of Independence, but their governments have been participating in EU-led negotiations for the past seven years. Both parties were informed that they had to normalize their relations as a precondition for EU membership.

Thaci said that a "border correction" could be part of the discussions. Some Serbian officials have suggested that an exchange of territories could help end the conflict.

Politicians from both countries have discussed the idea of ​​exchanging Presevo Valley, of ethnic Albanian origin, in southern Serbia with northern Kosovo populated by Serbs.

The idea, however, met with opposition from Germany and other European Union countries, who said they feared a trade between Kosovo and Serbia that could trigger requests for revision of the territory in other parts the unstable region of the Balkans.

Thousands of supporters of the opposition Kosovo Self-Determination Party marched Saturday in the capital Pristina to protest a possible border change. The demonstrators waved Albanian flags.

The opposition leader, Albin Kurti, said that he considered Thaci as a collaborator of Serbia and called for new elections.

"Such a grand demonstration is our response to the Thaci and Vucic agreements," Kurti said.

Thaci rejected both the border revisions based on ethnicity and possible land trade.

But he did not say how Serbia could be persuaded to give up the Presevo valley with nothing in return.

Three weeks ago, Serbian leader Vucic visited the lake in northern Kosovo where Thaci visited on Saturday.

NATO-led peacekeeping forces in Kosovo, a force known as KFOR, called for calm and restraint. They indicated that they would continue to monitor the situation along the Serbia-Kosovo border with ground patrols and helicopters.

Thaci's office issued a statement recognizing his visit to a border post and the lake.

"During the weekends, the head of state usually goes to the beauties of Kosovo," the statement said.

The governments of Pristina and Belgrade have expressed hope that the EU-led negotiations will lead to a legally binding agreement.

"The talks (with Serbia) that will continue will focus on peace and stability," Thaci said Saturday.

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This story has been corrected to show that the Serbian president was in northern Kosovo three weeks ago, not two.