Thousands of Serbs displaced by the 1999 fighting in Kosovo have camped on the border of the province to highlight their plight to a visiting UN mission.
The UN delegation is on a fact-finding trip about Kosovo's status
The delegation of 15 ambassadors from the UN Security Council is in Serbia on a fact-finding trip about Kosovo's long-term status.
The visit comes amid disagreement over the Serbian province's future.
The UN has run Kosovo since 1999, after Nato's bombing of Serb forces attacking ethnic Albanians seeking independence.
The organisers of the protest said they expected about 12,000 people, many of whom still live in camps.
"There are about 2,500 people here now and they keep arriving, with their own cars, with buses. We expect there to be some 12,000 during the day," Goran Savovic, deputy head of the Association of Exiled and Displaced Serbs, told Reuters news agency.
During its visit, the UN team has been meeting Serbian Prime Minister Vojislav Kostunica and President Boris Tadic in Belgrade before heading to Kosovo itself for talks with ethnic Albanian leaders.
They will also visit Serb enclaves and the divided city of Mitrovica.
The UN special envoy, Martti Ahtisaari, has come up with a plan which puts Kosovo on the path to independence.
The US and the European Union favour Mr Ahtisaari's plan, but Serbia and Russia are opposed.
The UN Security Council will soon vote on the plan.
Should Russia use its veto in the Security Council to block the Ahtisaari plan, Kosovo's ethnic Albanians might declare independence anyway, says the BBC's Laura Trevelyan in New York.
Such a move could encourage Serbia to try and claim the parts of Kosovo dominated by ethnic Serbs, our correspondent adds.