Ultranationalists were gathering Tuesday for a rally for Radovan Karadzic amid reports that the ex-Bosnian Serb leader's appeal of his extradition to the U.N. war crimes tribunal had not arrived at a Serbian court.

Karadzic's lawyer claimed he sent the appeal to the court by registered mail Friday before the midnight deadline. But the postal service said it doesn't have it and court spokeswoman Ivana Ramic said she didn't, either.

"There is nothing new. The appeal still isn't here," Ramic said.

Under Serbian law, if the appeal is not filed, or if it is sent by mail but doesn't arrive at the Belgrade court, the court's investigative judge can rule to extradite Karadzic to The Hague, Netherlands, without considering his objection.

Karadzic faces 11 charges at the U.N. tribunal, including genocide and conspiracy to commit genocide. Among the alleged crimes, he is accused of masterminding the 1995 slaughter of 8,000 Muslim men and boys in Srebrenica and the 3 1/2-year siege of Sarajevo that caused 10,000 deaths.

Hague spokeswoman Nerma Jelacic said the tribunal is ready to take Karadzic into custody whenever the Belgrade court rules.

Meanwhile, tensions were high ahead of an ultranationalists' anti-government rally planned in Belgrade for Tuesday evening to protest Karadzic's arrest.

The rally organizers — the right-wing Serbian Radical Party — said they were busing in Karadzic supporters from all over Serbia and Bosnia, where Karadzic is still revered as a wartime hero by nationalist Bosnian Serbs.

Karadzic's lawyer, Svetozar Vujacic, has predicted the government will try to whisk the war crimes suspect off to the U.N. tribunal before the protest.

In February, when the nationalists organized a rally against Western countries that recognized Kosovo's independence, the U.S. Embassy was partly burned and protesters went on a looting spree, smashing shops and McDonald's restaurants in Belgrade.

"The protest is against the treacherous and dictatorial regime" of Serbia's pro-Western President Boris Tadic, which arrested Karadzic last week after nearly 13 years on the run, Radical Party leader Aleksandar Vucic said.

The U.S. Embassy in Belgrade advised American citizens to avoid downtown, where the ultranationalist protest was scheduled, and to "exercise caution if within the vicinity of any demonstrations."

An embassy letter to U.S. citizens predicted between 25,000 to 100,000 people will attend the rally.

Officials say the war crimes suspect was captured July 21 in Belgrade, where he lived under the assumed identity of a health guru, with long white beard and hair, and large glasses. His lawyer claims that Karadzic was kidnapped in a Belgrade public bus July 18, and illegally held for three days by unknown captors.

Serbia's state television showed footage Monday of a bearded Karadzic that was purportedly made a month ago in a village close to Belgrade during his visit to an alternative fertility clinic. The informal gathering was called "talking about sex," the report said.

Serbia's new, pro-Western government hopes that Karadzic's arrest will strengthen the country's bid for European Union membership. Serbia had been accused of not searching for war crimes fugitives sought by the U.N. tribunal.