The appointment of Judge Rahman comes as a surprise, because another judge, Said al-Hammash, was next in line to succeed Judge Amin. Last week an Iraqi official petitioned the tribunal to bar Judge Hammash, saying he was a former member of Mr. Hussein's Baath Party. Judge Hammash has denied it, and the office of Prime Minister Ibrahim Jaafari directed the tribunal to pay no attention to the petition.

But the prospect of a political battle with the de-Baathification commission, which is charged with purging former high-level Baathists from public office and brought the accusation against Judge Hammash, appears to have swayed the tribunal, which moved him to another chamber.

Over all, the episode is seen as having created a troubling impression that the tribunal is unstable and vulnerable to political interference. Of the panel of five judges who started out on Mr. Hussein's trial on Oct. 19, three have now been replaced.

Judge Amin may yet return, Mr. Juhi said, because his resignation has not become formal.

"Judicial positions are not musical chairs," said Miranda Sissons, a senior associate at the International Center for Transitional Justice, who is observing the trial. "There has to be some measure of consistency to be sure the trial has integrity."

Judge Rahman, a Kurd, served as the chief judge of the Suleimaniya Criminal Court in the Iraqi region of Kurdistan before joining the Iraqi High Tribunal.

The tribunal's announcement came as an outbreak of insurgent violence on Monday left at least a dozen people dead across Iraq.

In Baghdad, a suicide bomber drove a car packed with explosives into a police checkpoint near the fortified Green Zone, killing a police officer and two civilians and wounding seven people, Interior Ministry officials said. A police patrol hit a roadside bomb, injuring two officers, and a member of Parliament, Jabir Khalifa al-Jabir, narrowly avoided an assassination attempt when gunmen opened fire on his car, killing his bodyguard and injuring his son.

South of Baghdad in the restive city of Mahmudiya, a suicide bomber detonated his vehicle outside a hospital, killing one civilian and wounding five others.

In Mosul, far to the north, gunmen shot and killed Jasim Muhammad, a prominent member of the Kurdistan Democratic Party, as he left his home. Elsewhere in the city, a roadside bomb detonated on an Iraqi Army patrol, killing a soldier and wounding another.

In the oil city of Kirkuk, a roadside bomb killed one policeman and wounded four others. In Baquba, insurgents detonated a bomb outside a house in the Tahrir district, wounding seven people, and then blew up a second bomb as the police and ambulances arrived, wounding six policemen, police officials said.

The American military said a soldier was killed while on a foot patrol in the southwest part of the capital. Military officials also said two American airmen were killed and one was wounded Sunday while escorting a convoy near Taji, Reuters reported.

The Association of Muslim Scholars, a hard-line Sunni Arab group, issued a statement on Monday saying that Interior Ministry forces had rounded up 32 people in western Baghdad and shot and killed two of them. Later, three of those abducted, all teenagers, were discovered not far away with signs of torture on their bodies, the statement said.

The Iraqi Islamic Party, Iraq's best-known Sunni Arab political party, also issued a statement describing the event, though it did not directly accuse the Interior Ministry.

The Interior Ministry said it was not aware of any raid in the area. Many Sunnis have been killed in western Baghdad in raids by gunmen dressed as security officers in recent months, but the identity of the killers is usually unknown.