THE HAGUE — Radovan Karadzic, a former Bosnian Serb leader, was convicted of genocide, war crimes and crimes against humanity by a United Nations tribunal Thursday for leading a campaign of terror against civilians that included the slaughter of 8,000 Bosnian Muslims at Srebrenica in 1995 and the nearly four-year siege of Sarajevo.

Karadzic, 70, was sentenced to 40 years in prison.

The UN International Criminal Tribunal for the Former Yugoslavia convicted Karadzic of genocide for the Srebrenica massacre, which aimed to kill “every able-bodied male’’ in the town and systematically exterminate the Bosnian Muslim community there.

Though acquitted on a second count of genocide, Karadzic was convicted of persecution, extermination, deportation, forcible transfer, and murder in connection with a campaign to drive Bosnian Muslims and Croats out of villages claimed by Serb forces during the country’s 1992-95 civil war.

In addition, Karadzic was found to have been “instrumental’’ in a campaign of sniping and shelling that terrorized the civilian population of Sarajevo, the Bosnian capital. And he was convicted of leading the taking of UN employees as hostages, to obstruct NATO from carrying out airstrikes on behalf of besieged Bosnian Muslim civilians.

The atrocities in Bosnia were a source of lasting regret for Bill Clinton, the president of the United States at the time, and spurred his administration to broker the Dayton peace accords in 1995 and, subsequently, support the NATO bombing of Belgrade in 1999 to prevent similar atrocities in Kosovo.

The decisions were read out by the presiding judge, O-Gon Kwon of South Korea, who took more than an hour as he calmly but precisely recounted a series of atrocities that were part of the most severe war crimes since World War II.

The judges took a year to deliberate — after being in session for 491 days, spread out over four years — reflecting the ambition of the prosecution and the complexity of conducting a criminal trial covering a civil war that left more than 100,000 people dead.

The case was likened to the postwar Nuremberg trials of former Nazi leaders.

Karadzic is the most senior political figure to be tried for events in the Bosnian war, which was part of the breakup of Yugoslavia. He had been charged with two counts of genocide, five counts of crimes against humanity and four counts of violations against the laws and customs of war.

Slobodan Milosevic, the Serbian president whose extreme nationalism instigated and enabled much of the fighting, died in March 2006 in his cell before the end of his trial at the UN tribunal. Ratko Mladic, who was Karadzic’s military chief during the campaign, is being tried separately here.

Karadzic acted as his own defense lawyer in the trial, portraying himself as a man of peace who was driven solely by his desire to protect Serbs.

In his closing statement, he said that he took “moral responsibility’’ for crimes committed by Bosnian Serb “citizens and forces,’’ but he denied having ordered killings and said he was not aware that a massacre would take place at Srebrenica, the Bosnian town that had been declared a UN safe haven and where 8,000 Muslim men and boys were killed.

Karadzic was also accused of organizing the three-year siege of Sarajevo, in which 12,000 residents were killed.

During the trial, he described himself as a “true friend to Muslims’’ who had tried to make them feel safe.

He brought 238 witnesses to attest to his innocence. He based his defense on the premise that the Bosnian war broke out because Serbs had no choice but to defend themselves against a Bosnian Muslim separatist regime that intended to create an Islamic state.

But the prosecution contended that Karadzic was the commander of a separatist Serb government bent on removing all non-Serbs from all areas of Bosnia that had been traditionally Serb.

Prosecutors presented electronic intercepts, written orders, video recordings and a long line of witnesses — fighters, politicians, peacekeepers, survivors of prison camps and rape victims — to demonstrate his central role in the conflict.

In a policy that came to be known as ethnic cleansing, hundreds of thousands of Muslims and ethnic Croats, largely Roman Catholic, were driven from their villages, their homes looted and mosques and churches demolished.

Men and boys were held in concentration camps, where prosecutors said thousands were tortured, killed, or died of starvation, and women were said to have been raped and used as sex slaves.

For the International Criminal Tribunal for the former Yugoslavia, the Karadzic verdict comes as it winds up its mandate to try those most responsible for the Balkan wars that raged in Croatia and Bosnia and that ended in Kosovo.