With the trial of Saddam Hussein set to begin in earnest Monday under extraordinary security, police said Sunday that they had foiled a plot to kill the chief investigating judge in the case.

The proceedings against Hussein, which started Oct. 19 and were suspended the same day, will go forward amid daily reminders of the turmoil that has followed his removal. The Canadian government announced Sunday that four aid workers, including two Canadians, had been kidnapped.

The Canadian Foreign Ministry did not identify the two or give any details of the kidnapping. The other two victims were widely reported to be an American and a Briton, but U.S. military officials said they had no reports of an American being kidnapped.

Meanwhile, a military vehicle carrying American lawmakers overturned on the way to the Baghdad airport Saturday, injuring two congressmen, according to a third congressman quoted Sunday by the Associated Press. Rep. Tim Murphy (R-Pa.) was airlifted to a military hospital in Germany for an MRI on his neck, and Rep. Ike Skelton (D-Mo.) was taken to a Baghdad hospital, said Rep. Jim Marshall (D-Ga.), who was traveling with them but was not hurt.

Police in the northern city of Kirkuk said they had arrested 10 men who had prepared car bombs and had written instructions from a former top deputy to Hussein to kill the judge who led the preparation of charges against the former dictator.

The arrests were made Saturday in a pre-dawn raid on a house near Kirkuk. Plainclothes policemen who stormed the house found a link between Hussein's former security apparatus -- the arrested men were identified as agents in the former government -- and al Qaeda in Iraq, the insurgent group led by Abu Musab Zarqawi, according to police Col. Anwar Qader Mohammed in Kirkuk.

One of the arrested men was the son of a Zarqawi deputy. Among literature and instructions from Zarqawi's group, Mohammed said, police found a letter and statements apparently prepared by Izzat Ibrahim Douri, Hussein's former vice president. Douri is said to be leading resistance by Hussein loyalists to Iraq's new government. He ordered the group in Kirkuk to kill the trial's chief investigating judge, Raeed Juhi, according to Mohammed.

The group also planned to set off bombs at crowded traffic circles in Kirkuk to distract attention from the trial, according to Mohammed.

On Monday, Hussein and seven co-defendants will be brought to a secret courtroom inside the heavily fortified Green Zone in Baghdad to hear the first testimony against them five weeks after their initial appearances in court. They are charged with ordering the executions of 148 men and boys from the Shiite Muslim town of Dujail after Hussein's motorcade came under fire near the town in 1982.

The trial has been jeopardized by threats and attacks. Two attorneys for Hussein's co-defendants have been killed, and another fled Iraq for his safety. The other defense attorneys agreed to resume work only after the U.S. and Iraqi governments promised to increase security around them.

Former U.S. attorney general Ramsey Clark and Najeeb Nauimi, a former justice minister of Qatar, announced Sunday that they were going to Baghdad to help in Hussein's defense. "A fair trial in this case is absolutely imperative for historical truth," Clark, 77, told the Reuters news agency in Amman, Jordan.

The tribunal was created by the Iraqi government -- with U.S. advice -- to hear the charges against Hussein and his former lieutenants. Prosecutors have said they might bring more charges against Hussein, including the mass slayings of ethnic Kurds and Shiite Muslims, if he is not hanged first.

Hussein's lawyers have challenged the legitimacy of the court, saying it is under the control of the United States. And human rights groups have raised questions about proposals to present testimony from anonymous witnesses and recorded testimony from witnesses now dead.

At least four roadside bombings were reported in Iraq Sunday. The blasts occurred in Baghdad, Mosul, Baqubah and Riyadh and killed three civilians and a policeman, according to news agencies and local reports.

U.S. military authorities said a Marine who died in western Iraq was the seventh serviceman killed in the country in four days. Authorities did not release the name of the Marine, who was killed by an improvised bomb.

Protesters in the city of Najaf hold pictures of relatives killed under Saddam Hussein's government as they burn an effigy of the former president.