The Serb National Council, which organised the protest, said it was targeting the US embassy because of Washington's support for Kosovo's pro-independence ethnic Albanians.
Barnaby Phillips, Al Jazeera's correspondent in Belgrade, said that about 20,000 people participated in the rally.
Event organisers said 30,000 to 35,000 people attended.
|"It's a question of national identity. Serbia was born in Kosovo and it is our heart and soul" |
Dejan Backovic of the Socialist party
The rally was swelled by Serbs who had travelled to Belgrade by car and bus from Mitrovica, a town in the north of the province where Serbs still form a majority.
The rally was backed by nationalist parties including the Socialist party of Slobodan Milosevic, Serbia's former president.
"It's a question of national identity. Serbia was born in Kosovo and it is our heart and soul," Dejan Backovic of the Socialist party told Phillips.
Belgrade and most Serbs see Kosovo as the cradle of their history, culture and religion.
The Democratic Party of Serbia, of which Vojislav Kostunica, Serbia's prime minister, is a member, also supported the protest.
Before the rally, Milan Ivanovic, Serb National Council leader, told Roderick Moore, the deputy US envoy to Belgrade, that the Ahtisaari plan was "unacceptable and biased," the state-run Tanjug news agency reported.
Moore responded that Washington supported the plan as the basis for ongoing negotiations between Serbian and Kosovo Albanian representatives.
Earlier, Miguel Angel Moratinos, the chairman of the Organisation for Security and Co-operation in Europe (OSCE) and Spain's foreign minister, urged Serbia to be "constructive" in the final round of UN-backed talks.
"I have encouraged the Serbian leaders to be constructively involved in the Kosovo consultations," said Moratinos after meeting with Kostunica.
Boris Tadic, Serbia's president, and Vuk Draskovic, acting foreign minister, also attended the meeting with Moratinos.