BAGHDAD, Iraq -- An Iraqi government search of a detention center in Baghdad operated by Interior Ministry special commandos found 13 prisoners who had suffered abuse serious enough to require medical treatment, U.S. and Iraqi officials said last night.

An Iraqi official with firsthand knowledge of the search said at least 12 of the 13 prisoners had been subjected to "severe torture," including courses of electric shock and episodes that left them with broken bones.

"Two of them showed me their nails, and they were gone," the official said on condition of anonymity because of security concerns.

A government spokesman, Laith Kubba, said last night that any findings at the prison would be "subject to an investigation" but declined to comment on the allegations.

The revelation comes as the nation prepared for parliamentary elections on Thursday.

The site, which was searched last Thursday, is the second Interior Ministry detention center where cases of prisoner abuse have been confirmed by U.S. and Iraqi officials.

U.S. troops found the first site last month when they entered an Interior Ministry building in central Baghdad to look for a Sunni teen-ager who they believed had been detained, officers said at the time. Several prisoners at that site appeared to have suffered beatings, and many were emaciated, U.S. and Iraqi officials and witnesses said.

International law, including the U.N. convention against torture, bans torture in all cases. U.S. Ambassador Zalmay Khalilzad issued a sharp public rebuke of the Iraqi government after the secret prison was discovered last month, demanding in a statement that all detainees nationwide be treated in accord with human rights.

Prime Minister Ibrahim al-Jaafari, under heavy pressure from Mr. Khalilzad and Gen. George Casey, the top U.S. commander in Iraq, ordered a nationwide investigation of detention centers after that discovery. The prison investigated Thursday was the first center examined as part of the government-ordered inquiry.

Lt. Col. Guy Rudisill, a spokesman for U.S. military detention issues, said U.S. authorities had been aware that the prison searched Thursday existed. U.S. forces had not known about the previous facility.

Prison inspectors from the Ministry of Human Rights and representatives of other ministries participated in the commando prison search, the ministry said in a statement. Authorities did not say whether any Americans were involved in the inquiry.

Investigators said they found 625 prisoners at the center but declined to give details about them. Most of the detainees found at the secret prison last month were Sunni Arabs who had been picked up by the Shiite-dominated Interior Ministry forces.

U.S. diplomatic and military officials said Iraqi officials were leading the investigation and declined to offer further comment.

Torture was routine at prisons under former president Saddam Hussein. U.S. forces in Iraq came under international criticism for abuses at the Abu Ghraib prison in which military guards photographed themselves humiliating naked Iraqi detainees. The Iraqi government, led by Shiite parties with strong ties to Iran, has strongly rejected allegations of Iranian intelligence involvement in Interior Ministry prisons.

The Iraqi Interior Ministry has a significant presence of former militia members and other members of the Shiite Supreme Council for Islamic Revolution in Iraq, which is the largest party in the government.

The country's Sunni minority has accused the Interior Ministry of taking a lead role in severe abuses, including alleged death squads that have targeted Sunnis in particular.

With just four days to go until parliamentary elections, the Iraqi electoral commission said yesterday that it had found irregularities in voter registration in the volatile northern oil city of Kirkuk, The New York Times reported.

The discovery was the first instance of an election irregularity announced by the commission as the country prepares for the vote on Thursday.

The commission said experts conducting an audit of voter lists found that there had been an unexpected surge in voter registration in the area. When the experts scrutinized the voter registration forms, the commission said in a written statement, they found that many had been filled out incorrectly. Some had missing signatures and others had more than one signature. In some cases, the same name appeared on several forms.

Adel al-Lami, the director general of the Iraqi electoral commission, said in an interview with The Times yesterday that in his view the voter registration irregularities were technical errors and not politically motivated.

Kirkuk is considered one of the most potentially incendiary cities in Iraq, because of its diverse ethnic and religious mix and its oil resources. The area, north of Baghdad, has 10 percent to 20 percent of the country's oil reserves. As a result, several competing groups -- Kurds, Turkmens and Arabs -- claim dominance over the city.

The Interior Ministry laid out security plans yesterday for the period surrounding the elections. The measures are similar to ones put in place during last January's elections and during the constitutional referendum in October. The government will shut down from Dec. 13 to Dec. 17, as a national holiday, and a nightly curfew of 10 p.m. to 6 a.m. will be in place on those days. In addition, civilians will not be allowed to carry guns even if they have a permit.

Iraqi forces will also clamp down on movement across the country's borders and on travel between provinces.

Advance voting is scheduled to take place today in hospitals and prisons.

The U.S. military said yesterday that a soldier was killed in a roadside bomb explosion in Baghdad. At least 2,144 U.S. troops have died in the war.

Separately, Iraqi and British officials said yesterday they had no word on the fate of four Christian peace activists, more than a day after the expiration of a deadline set by kidnappers to kill them if all prisoners weren't released.