ISKANDARIYAH, Iraq - A car bomb exploded this morning at a police station south of Baghdad as dozens of would-be recruits lined up to apply for jobs, and a hospital official said at least 50 people were killed and 50 others wounded.

U.S. officials in Baghdad put the figure at 35 killed and 75 wounded but said the figure could be higher since Iraqi authorities were handling the investigation. The local Iraqi police commander, Lt. Col. Abdul Rahim Saleh, said the attack was a suicide operation, carried out by a diver who detonated a red pickup as it passed by the station.

"I am sure it was a vehicle," Saleh said. "We found its engine. It was a suicide operation and a cowardly act."

The blast followed warnings from occupation officials that insurgents would step up attacks against Iraqis who work with the U.S.-led coalition, especially ahead of the planned June 30 transfer of sovereignty to a provisional Iraqi government.

On Monday, U.S. officials said a letter seized last month from an al-Qaida courier asked the terrorist leadership to help foment civil war here to undermine the coalition and the future Iraqi leadership.

The purported author of the letter was Abu Musab al-Zarqawi, a Palestinian-Jordanian suspected of al-Qaida links. The author boasted of having organized 25 suicide attacks in Iraq.

U.S. paratroopers from the 82nd Airborne Division sealed off the area around the station and refused to allow journalists near the blast site about 30 miles south of Baghdad.

In Baghdad, Lt. Col. Dan Williams, a coalition spokesman, said no U.S. or other coalition forces were hurt.

Hospital director Razaq Jabbar said his facility had received 50 dead and 50 injured - all believed to be Iraqis. He said he had heard that three others died at another hospital.

"This figure might increase," he said. "There were some body parts that haven't been identified yet. Some more bodies may be trapped under the rubble."

Policeman Wissam Abdul-Karim said he was standing in front of the nearby courthouse when "I heard a very strong explosion" and "the blast threw me on the ground."

"It was the day for applying for new recruits," Abdul-Karim said. "There were tens of them waiting outside the police station."

Security for the facility included a checkpoint surrounded by sandbags and barbed wire, Abdul-Karim said.

Hussein Mohammed, 18, said he was standing in the public market when he heard a tremendous explosion about 9:15 a.m. Another witness, who refused to give his name, described the blast as "really strong" and said body parts littered the street near the station.

"There was not one body in one piece," he said.

Jabbar said some of the victims were policemen "but many more were civilians applying for jobs, and passers-by."

Insurgents have mounted a string of car and suicide bombings in recent weeks. The deadliest so far has been in the northern city of Irbil on Feb. 1 when two suicide bombers blew themselves up at two Kurdish party offices celebrating a Muslim holiday, killing at least 109 people.

On Jan. 18, a suicide car bomb exploded near the main gate to the U.S.-led coalition's headquarters in Baghdad, killing at least 31 people.

A car bomb exploded Aug. 29 outside a mosque in the Shiite Muslim holy city of Najaf, killing more than 85 people, including Shiite leader Ayatollah Mohammed Baqir al-Hakim.

Meanwhile, the Baghdad Convention Center, which houses the U.S. military press center and other coalition facilities, was evacuated today after bomb-sniffing dogs detected something suspicious, Williams, the coalition spokesman, said. The center later reopened.

On Monday, a suicide bomber walked up to the house of brothers Majid and Amer Ali Suleiman in Ramadi, west of Baghdad, and detonated explosives strapped to his body, witnesses said.

Three guards were seriously injured but the brothers - who are among the city's most prominent tribal leaders working with coalition forces - were unhurt.

The bomber had approached the house earlier when the brothers were receiving callers, and was told to leave, the witnesses said.

Insurgents have repeatedly warned Iraqis not to cooperate with the Americans. The most recent threats were contained in pamphlets circulated in Ramadi and nearby Fallujah by a purported coalition of 12 insurgent groups.

Ramadi and Fallujah are located in the Sunni Triangle, a major center of resistance to the U.S.-led occupation.

It was the second instance of a suicide bomber carrying out an attack with explosives on his body, although suicide car and truck bombings are not unknown.

Also Monday, defense officials in Washington said American forces in Iraq have detained one of the remaining most-wanted members of Saddam Hussein's government.

Muhsin Khadr al-Khafaji, No. 48 on the 55 most-wanted list, was turned over last weekend to U.S. troops in the Baghdad area, the officials said, speaking on the condition of anonymity.