Momcilo Krajisnik

 Robertson said soldiers detained Momcilo Krajisnik , who was under a sealed indictment by the International Criminal Tribunal for Yugoslavia for war crimes.

Richard Holbrooke, U.S. ambassador to the United Nations, applauded the arrest of Krajisnik. "This is a great day for those of us who have fought for a single multiethnic country in Bosnia," said Holbrooke, who worked to bring about the Dayton peace accords for Bosnia in 1995.

Holbrooke called Krajisnik "one of the worst of the people in the region -- a racist, a separatist, a war criminal ..."

Witnesses to the arrest said NATO troops with the Bosnian peacekeeping force detained the Serb leader after forcing open a door to his home in Pale, southeast of Sarajevo, with explosives. The French Defense Ministry in Paris said French troops made the arrest.

NATO said Krajisnik was being processed for transfer to The Hague, Netherlands -- where the Yugoslav war crimes tribunal is located.

"They took my dad away," Krajisnik's son Milos, 21, said. "Some of them spoke Serbian, some English but mostly French." He said he and his brother, Njegos, 19, were tied and their faces turned toward the floor during the arrest.

Robertson said that between July 1, 1991, and December 31, 1992, Bosnian Serb forces, under the direction and control of Krajisnik, then a leading member of the Serbian Democratic Party of Bosnia and Herzegovina, secured control of several municipalities that had been proclaimed part of the Serbian Republic of Bosnia and Herzegovina.

"He is accused of genocide, crimes against humanity, violations of the laws and customs of war, and grave breaches of the Geneva Conventions, including murder, willful killing, extermination, complicity in genocide, deportation, and inhumane acts," said Robertson in a statement.

"This arrest -- the sixth since I became Secretary General -- represents the capture ... of the highest ranking person indicted for war crimes in the former Yugoslavia thus far. It is good news for justice, and good news for the people of Bosnia and Herzegovina.

"To those individuals who remain at large I will repeat what I have said many times before: The net is closing. It is time to turn yourselves in," said Robertson.

Still at large are Karadzic, the No. 1 war crimes suspect in Bosnia, and his top general, Ratko Mladic. Krajisnik, Karadzic's senior aide for most of the Bosnian war, replaced him as the leader of Bosnia's Serbs after Karadzic was forced to give up public functions because of his indictment by the war crimes tribunal.

In The Hague, chief prosecutor Carla Del Ponte also urged the arrest of Karadzic, "who should stand trial jointly with the accused, Momcilo Krajisnik."

Holbrooke called on Karadzic to surrender: "This is a very strong signal that the handwriting's on the wall. He'd better read it. He ought to voluntarily turn himself in."

Holbrooke added, "He faces two futures now -- to spend the rest of his life on the run or else he can defend himself in the International Court of The Hague."

Tribunal spokesman Paul Risley said Krajisnik was one of "the individuals who ran illegal operations that resulted in the deaths of thousands of Bosnians."

"He was present at every meeting where political and military actions were decided upon that resulted in deportations, illegal arrests, ethnic cleansing and the deaths of thousands of Bosnians," Risley said.

Krajisnik became speaker of the Bosnian parliament in 1990, before Serbs walked out and the war began.

The dour, beetle-browed Krajisnik was the powerful speaker of the separatist Bosnian Serb parliament, which repeatedly stalled or rejected agreements brokered by international mediators to end the 1992-95 war.

Under the leadership of Krajisnik and Karadzic, Serb forces laid bloody siege to Sarajevo, purged Moslem and Croat populations from Serb-held territory in "ethnic cleansing" campaigns and sacked two U.N.-designated "safe areas," apparently executing thousands of unarmed men caught while fleeing the enclave of Srebrenica.

Krajisnik served in 1996-98 as the Serb member of Bosnia's first post-war, multi-ethnic collective presidency, but analysts said he used his position primarily to thwart any reintegration between Bosnian Serb and Moslem-Croat entities in Bosnia.

In 1998 Krajisnik lost his bid for re-election, defeated by a relatively moderate Serb leader, Zivko Radisic.

At the time, Serb moderates were wresting control of republican affairs from hard-line nationalists like Krajisnik who were refusing to cooperate with international peace coordinators, jeopardizing reconstruction aid.

Krajisnik's early post-war strength stemmed from his control of hard-line police and municipal authorities, which stirred accusations of profiteering similar to black-marketeering that allegedly enriched him and his associates during the war.

After leaving the Bosnian presidency, Krajisnik remained a member of the board of the Serb Democratic Party founded by Karadzic a decade ago. But party officials said he was no longer politically active, focusing instead on his business interests and generally keeping a low profile.

Aides describe the widower and father of three as conservative and pious. He considers separation based on ethnicity and religion to be natural.

Mirko Banjac, a ranking official of Krajisnik's Serb Democratic Party, expressed concern at the arrest, adding: "We have the right to demand an explanation."

Amor Masovic, head of the Muslim commission for missing persons, described Krajisnik as "one of the masterminds of the genocide and ethnic cleansing in Bosnia."

The arrest, coming ahead of next weekend's Bosnian municipal elections, heightened concerns that the voting could be marred by violence. It also might boost radical nationalist Serbs opposed to reconciliation in Bosnia.

The arrest will "radicalize the political environment," said Serb moderate Mladen Ivanic.

The Associated Press and Reuters contributed to this report.