Marti Ahtisaari, a UN envoy for talks on Kosovo, has proposed that the Balkan province be given an internationally supervised sovereignty.
He said earlier that talks are to end by March 10, after which the matter will return to the UN Security Council.
The demonstrators protested Ahtisaari's plan granting Serbia's Albanian-populated region independence, which Serbian politicians see almost as a foregone conclusion.
They appealed to Russia to use all diplomatic means to prevent the creation of yet another state in the Balkans, carrying placards and posters reading "Russia, help us!"
The rally was organized by the Serbian National Council, one of the largest political organizations of Kosovo Serbs, and was supported by the Serbian Radical Party, the Socialist Party of Serbia and the Democratic Party of Serbia.
Belgrade and Pristina held talks on Ahtisaari's proposal in Vienna last week, but no breakthrough was made.
Belgrade has rejected proposals to give independence to the region, which has been under a UN protectorate since 1999 after U.S. air raids conducted to end alleged ethnic cleansing by Serbian troops.
Serbian authorities say they are willing to grant Kosovo broad autonomy, but will never let the province secede from Serbia.
Albanian leaders have said Kosovo's independence is the only option for them.
Russia, a traditional ally of fellow Slavic Serbia and a veto wielding Security Council member, has been opposed to internationally backed plans to grant sovereignty to Kosovo, also arguing it would set a precedent for the breakaway regions in the former Soviet Union it is believed to support - Georgia's Abkhazia and South Ossetia, and Moldova's Transdnestr.
