Nato troops have sealed the northern borders of Kosovo after Serbs angry at its weekend declaration of independence ransacked two crossings.
Serbs set fire to UN and police vehicles at Jarinje
Hundreds of protesters torched customs and police posts at Jarinje and Brnjak, manned by UN and Kosovo police.
Closing the borders will infuriate both Kosovo Serbs and Serbia's government, says the BBC's Nick Thorpe in Kosovo.
The move cuts Kosovo's mainly Serb north off from Serbia, the country to which they insist they still belong.
Nato said it was shutting the crossings, initially for a 24-hour period.
Bulldozers and explosives
The protesters arrived in convoys of cars and buses in what our correspondent says was clearly a carefully planned and coordinated action.
Kosovo police and UN customs officials were forced to withdraw to a nearby tunnel as the crowds used bulldozers and explosives to demolish the border posts, according to witnesses.
Nato-led peacekeepers from K-For were called in and US, Estonian and French troops sealed the two border crossings.
Serbia's minister for Kosovo, Slobodan Samardzic, said his government intended to take control of the customs posts to establish what he called the fully authority of Serbian statehood.
The attacks on the crossings and the Nato response put the international community on a collision course with both Kosovo Serbs and the Serbian government, our correspondent says.
He says that what is emerging on the ground is a second Kosovo.
Students in the Serb-dominated town of Mitrovica are organising daily protests at 12.44 pm, referring to UN Security Council resolution 1244 under which Serbia insists it still has sovereignty of Kosovo under international law.
In Pristina, Dutch diplomat Pieter Feith began his task as the EU's special representative to Kosovo.
Mr Feith - who will head the international civilian office due to take over from the UN - insisted that the EU mission would be deployed throughout Kosovo despite Serb hostility.
"We need to reach out to the Serb communities here in Kosovo and we are, of course, in touch with Belgrade," he said.
A 2,000-strong EU police and justice mission will take shape in the coming weeks.
Russia has sharply criticised Brussels' role in Kosovo. Foreign Ministry spokesman Mikhail Kamynin said that by "pursuing the unilateral scenario of solving the Kosovo problem... the European Union encourages separatism in the world".
He was speaking after EU foreign policy chief Javier Solana became the first international statesman to visit Kosovo since its independence declaration on Sunday.
STANCE ON RECOGNITION
For: Germany, Italy, France, UK, Austria, US, Turkey, Albania, Afghanistan
Against: Russia, Spain, Romania, Slovakia, Cyprus
The US, Britain, France, Germany and Italy have all recognised the new state, but others have not.
The UN Security Council is divided over how to respond to Kosovo's move, and it has failed to agree on any action.
China has expressed its deep concern and official media in Beijing announced that a further small deployment of UN peacekeepers would be sent to Pristina next month.
On Monday, the Serbian parliament passed a resolution condemning Kosovo's declaration of independence.
The resolution also formally annulled the acts of the government in Pristina, saying Belgrade's sovereignty over Kosovo was guaranteed by the UN and international law.
In a separate move, Serbia has recalled its ambassadors to the US, France and Turkey because those countries had recognised Kosovo's independence.
EU member states set aside differences over the recognition of Kosovo earlier this week by stressing that it was not a precedent for separatists elsewhere.
Spain and several other member states have withheld recognition because of concerns about separatist movements within their own borders.
Serbian security forces were driven out of Kosovo in 1999 after a Nato bombing campaign aimed at halting the violent repression of ethnic Albanian separatists.
The province has been under UN administration and Nato protection since then.