Thousands of Serbs have demonstrated outside the U.S. Embassy in Belgrade, protesting the United Nations envoy to Kosovo's plan for the future of the breakaway Serbian province.

At least 20,000 people joined the protest accusing the United States of sponsoring the U.N. plan and urging Russia to veto it.

The top U.S. diplomat in Belgrade received protest representatives, telling them the plan, proposed by U.N. envoy Martti Ahtisaari, will guarantee the rights of all the people of Kosovo, including its Serbs.

In Moscow, Russian Foreign Minister Sergei Lavrov again said his country opposes any plan not acceptable to both sides.  He expressed concern that the Ahtisaari proposal ignores Belgrade's legitimate concerns.

The plan offers Kosovo self-government and membership in international organizations, but falls short of the outright independence that its ethnic Albanians demand.

Serbia rejects it as a violation of its sovereignty and territorial integrity.

The protest Tuesday came as representatives of Serbia and Kosovo's ethnic Albanian majority were resuming talks in Vienna on the plan.

Earlier, Spanish Foreign Minister Miguel Angel Moratinos, whose country holds the rotating chairmanship of the Organization for Security and Cooperation in Europe, urged both sides to work for compromise on the issue.  He spoke in Belgrade after meeting with Serbian President Boris Tadic.

Some information for this report was provided by AFP, AP and Reuters.