THE HAGUE, Netherlands — The U.N. war-crimes tribunal at The Hague finally took custody of Radovan Karadzic today — 13 years after the former Bosnian Serb leader went on the run — putting him in a jail where he was free to mingle with dozens of former allies and enemies.

Serbia handed over Mr. Karadzic overnight, flying him in a government jet to the Netherlands from Belgrade to await trial on charges of waging genocide against non-Serbs during the 1990s Balkan wars. His new home is a special wing of a Dutch prison reserved for people accused of war crimes in the Balkan wars.

“The arrest of Radovan Karadzic is immensely important for the victims who had to wait far too long for this day,” tribunal prosecutor Serge Brammertz told reporters at a news conference at the court. “[It also showed] that there is no alternative to the arrest of war criminals and that there can be no safe haven for fugitives.”

Mr. Karadzic’s arrival in The Hague marked the end of a 13-year effort by the Yugoslav war crimes tribunal to get hold of its most wanted war criminal. He is accused of responsibility for the deaths of tens of thousands of people and the suffering of hundreds of thousands of Bosnian Muslims and Croats.

Legal experts consider Mr. Karadzic the most important figure in the war crimes committed in Bosnia, exceeding the role played by the late Yugoslav president, Slobodan Milosevic, whose own trial ended inconclusively when he died in 2006 in the same U.N. jail.

During the 1992-1995 Bosnian war, Mr. Karadzic was known as the urbane, intellectual face of a monstrous regime blamed for the worst atrocities in Europe since World War II. According to his indictment, he and other senior Bosnian Serb leaders unleashed ethnic cleansing campaigns to drive Muslims and Croats out of land he considered part of a “Greater Serbia.” The terror reached its climax at Srebrenica.

Prosecutors allege Mr. Karadzic masterminded atrocities including the 1995 massacre of 8,000 Muslim men and boys at the Bosnian town of Srebrenica, the deadly siege of Sarajevo, and the detention of tens of thousands in 20 concentration camps where many were tortured, starved, and sexually abused.

Serbian authorities say they arrested Mr. Karadzic on July 21 at the capital Belgrade, where he had been living under a false identity and practicing alternative medicine.

At Belgrade, Mr. Karadzic’s lawyer, Sveta Vujacic, said his client will postpone entering a formal plea for 30 days, the maximum allowed under court rules.

Mr. Karadzic’s extradition stalled while a Belgrade court waited for an appeal, but Mr. Vujacic said he never filed one.

Last night, hours before Mr. Karadzic left Belgrade, about 15,000 Serb extremists rallied in a main square in the Serb capital demanding a halt to the extradition. Several hundred hooligans separated from the group and hurled stones and burning flares at riot police.

Later, police fired tear gas and rubber bullets at demonstrators, trying to push them from the square. Belgrade’s emergency clinic said it treated 51 policemen and 23 civilians injured in clashes. The city’s military clinic reported treating three policemen and three civilians.

Today, the extremist Serbian Radical Party accused pro-Western president Boris Tadic of “provoking and organizing” the violence. The party’s leader, Tomislav Nikolic, says Mr. Tadic wants to “wipe out” the Radicals and “take Serbia into a war.”

In The Hague, the war crimes tribunal said Mr. Karadzic will be summoned before a judge tomorrow and asked to enter a plea on each of 11 counts, including genocide, extermination, and persecution.

“It will take some months before the prosecution and the defense are ready to start this trial,” the Belgian prosecutor, Mr. Brammertz, said.

A tribunal spokeswoman, Nerma Jelacic, said the court will ensure Mr. Karadzic’s “well being and right to a fair trial as much as possible and in accordance with the highest international standards.”

Mr. Karadzic is being held in the same high-security prison that once housed his former mentor Milosevic and where other former allies and enemies are also in custody. The war-crimes tribunal jail is in a separate wing of a prison at Scheveningen, a coastal suburb of The Hague.

The center, which has 84 cells, currently has 37 other detainees, all of them alleged Yugoslav war criminals. Each cell, measuring 17 by 10 feet, has a shower, toilet, sink, and desk.

Cell doors are left open most of the day, except for a brief midday period to allow for a change of the guards. Prisoners may have computers, but are not allowed Internet access. They also receive Dutch, German, Belgian, and French television channels, as well as satellite reception in their own language.

Courses in arts, languages, or sciences are available. They share a gym, outdoor courtyard, library, and a recreation room for darts, table tennis, and board games. They have access to a doctor, nurse, and psychiatrist, and to a hospital in the adjoining Dutch prison.

The tribunal declined to give details about Mr. Karadzic’s transfer, citing security in future cases. But it confirmed his arrival shortly after a helicopter landed behind the high wall of the jail while another helicopter hovered overhead.

Mr. Karadzic’s top commander, Ratko Mladic, also accused of genocide, remains at large.

Mr. Brammertz praised Serbia’s new pro-Western government, saying it deserved “full credit” for the arrest. He said he hopes the new cooperation will shortly lead to Mr. Mladic’s arrest and that the two men could be tried together.

“It is clear that if General Mladic is arrested in the near future that this will be a serious option,” he said.

Bosnia’s international administrator lifted the travel ban against Mr. Karadzic’s family after Serbia handed him over to the war crimes tribunal. The family had been banned the family from leaving Bosnia because of suspicions they helped Mr. Karadzic elude capture.