Up to 1,000 men, some suspected of being members of the Serbian Ministry of Interior police, crossed into northern Kosovo yesterday amid rising fears that minority Serbs living in the new state's north would attempt effectively to partition Kosovo along the Ibar River.

After two days of rapidly rising tensions between Serbs and Albanians following Kosovo's declaration of independence from Serbia on Sunday, eyewitnesses counted close to 200 cars and buses crossing from southern Serbia full of men.

In a day of high drama, masked Serbs also torched two border posts separating Serbia from Kosovo, located at Jarnije and Banja about 18 miles north of Mitrovica, with bulldozers and plastic explosives.Nato troops later closed down the roads leading to the checkpoints, cutting off the only link between northern Kosovo and Serbia.

Several incidents were reported overnight, including masked attackers throwing grenades at UN and Albanian-owned buildings. No one was reported injured.

UN and Nato officials seemed to have been taken largely by surprise by yesterday's events, which saw KFor troops being sent in to rescue personnel trapped at the border posts.

Protesters also tipped over metal sheds that housed Kosovo's customs service and sent them sliding down a hill and into a river. They also vandalised and set fire to passport control booths. "It was very dangerous and the police had to withdraw and call for help from Nato peacekeepers," said Veton Elshani, a spokesman for Kosovo's multi-ethnic police force.

In Belgrade, the Serbian government minister for Kosovo, Slobodan Samardzic, hailed the attacks, saying: "Today's action is in accordance with the general [Serbian] government policies.

"Belgrade has the intention to take over the customs in northern Kosovo," Samardzic told private B92 television. "The customs points were intended to become part of [Kosovo's] state border and we are not going to let that happen."

The attacks and arrival of the convoys from Serbia were an ominous reminder of the enduring potential for violence in the new nation of Kosovo as it embarks on European-supervised independence.

The EU's new envoy, Pieter Feith, arrived in Pristina yesterday, accompanied by the EU foreign policy chief, Javier Solana, to mark the beginning of Kosovo's new era. Feith will lead a "stability mission" of 1,800 EU police and legal experts who will run the country's court system for the next few years.

In the coming days, Feith, a Dutch diplomat with extensive Balkan experience, will also take over leadership of the International Civilian Office, which will give him the power to overturn legislation and sack Kosovan officials.

European officials insisted he would try to keep a low profile and use his powers only in extreme circumstances when, for instance, the country's democracy or minority rights were in jeopardy.

Yesterday's incidents were an illustration of the central problem facing Feith and the Albanian majority government in Pristina - the refusal of Kosovo's Serb minority and the Belgrade government to acknowledge the former province's sovereignty.

Elsewhere in the Serb-dominated north witnesses also spoke of police stations once occupied by the joint Serb-Albanian Kosovo police service now displaying the Serb flag. The fears that Serbia is intending to put its police force into the north follow rumours being circulated among Kosovo police service officers in Mitrovica that Serb members are planning to resign en masse in the coming days.

The attacks and the arrival of a convoy from Serbia come as further demonstrations are planned against the declaration of independence in Mitrovica and Serbian cities this week.