PRISTINA -- Tens of thousands of people took to the streets of the capital of Kosovo on September 29 to protest a possible territory swap with Serbia.

Supporters of Kosovo's opposition Self-Determination Party held banners and national Albanian flags as they marched through Pristina toward Skanderbeg Square, chanting "No bargaining with national land!"

Kosovo President Hashim Thaci has mentioned a "border correction" that would bring southern Serbia's Albanian-dominated Presevo Valley into Kosovo.

Some Kosovar and Serbian officials have discussed a deal based on where ethnic minorities are concentrated, such as the Presevo Valley and the northern region in Kosovo where many Serbs live.

Although some EU and U.S. officials have said they support the exchange of territories, Germany and many analysts have said it is a bad idea that could renew old ethnic hostilities throughout the Balkans.

The Self-Determination Party has accused Thaci of bargaining Kosovo's territory with Serb President Aleksandar Vucic.

“Kosovo cannot be partitioned, independence is non-negotiable,” Kurti told protesters.

Earlier in the day, Thaci visited an area around the Kosovo side of Gazivode Lake in the country's Serb-dominated north.

Kosovo special police were deployed in the area for the visit, prompting Serbia to put security forces on alert, according to Marko Djuric, Serbian director for the Office of Kosovo.

Vucic, who visited the same area on September 8, was due to address the nation later in the evening.

There was no immediate reaction from Kosovo.

Maja Kocijancic, a spokeswoman for EU foreign policy chief Federica Mogherini, called on all sides to "react with calm and restraint."

Echoing that appeal, NATO-led peacekeepers in Kosovo, a force known as KFOR, said they would continue monitoring the situation along the Serbia-Kosovo border with ground patrols and helicopters.

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.

Serbia lost control over Kosovo in 1999 after a NATO bombing campaign to stop the killing and expulsion of Albanians by Serb forces during a two-year counterinsurgency war.

Kosovo declared independence in 2008 and has been recognized by more than 100 countries, but not by Serbia.

The two sides in 2013 committed to EU-mediated talks to resolve their differences, but little progress has been made.

Based on reporting by AP and Reuters