By Matt Robinson

MITROVICA, Serbia, Dec 18 (Reuters) - Protesting Serbs in Kosovo warned the European Union on Tuesday they would reject its planned supervisory mission if the Albanian majority declares independence from Serbia early next year.

Several thousand flag-waving Serbs gathered in the ethnically divided town of Mitrovica, a day before the U.N. Security Council debates the failure of Serb-Albanian negotiations and the possible way forward.

With Western backing, Kosovo's 90-percent Albanian majority is preparing to declare independence within weeks, setting up a showdown with Serbia and its big-power backer Russia.

The EU has committed to deploy 1,800 police officers, judges and administrators to oversee the fledgling state, but the mission faces rejection in the north, dominated by just less than half of Kosovo's remaining 120,000 Serbs.

"The EU cannot come here to implement the plan of that Finn, which has already been rejected by the U.N. Security Council," hardline Kosovo Serb political leader Marko Jaksic told the crowd, in reference to U.N. envoy Martti Ahtisaari.

Russia has blocked U.N. adoption of Ahtisaari's plan for 'supervised independence', but the United States and most EU members say they will move ahead with its implementation after Serb-Albanian negotiations ended this month in failure.

NATO allies with 16,000 troops in Kosovo are concerned about a backlash by Serbs in the north, and a possible bid to break away that could spark violent unrest.

Several hundred Serbs, yelling "Go Home", filed past the offices of a small, advance EU team preparing the ground for a sub-office of Kosovo's future international overseer.

The U.N. mission in Kosovo last week accused Serbia of "provocation" after the government opened an office in north Mitrovica. The region, which adjoins Serbia proper, has resisted U.N. efforts to integrate it with the rest of the province.

Serbian Prime Minister Vojislav Kostunica, a close ally of Jaksic, last week said the EU mission would be "unlawful".

Kosovo, to many Serbs the cradle of their religion and identity, has been run by the United Nations since 1999, when NATO drove out Serb forces to halt the killing and ethnic cleansing of Albanians.

The West now sees independence as the best option for stability in the Balkans, but Russia says it will set a precedent for other separatist regions. (Editing by Giles Elgood)

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