BAGHDAD, Iraq — Supporters of Muqtada al-Sadr, the militant Shiite Muslim cleric responsible for two uprisings against the United States last year, clashed with Iraqi security forces in the southern city of Kufa yesterday, on a day of widespread violence in Iraq.

A mass grave was found at a dump near the Shiite slum, Sadr City, that is al-Sadr's stronghold in the capital, and two suicide car bombs struck a market and a police checkpoint, killing at least 23.

Yesterday's violence underscored the weakness of the new Iraqi government in the face of widespread challenges to its authority.

As a sense of crisis in the country builds, the new government has been paralyzed by indecision and riven by factional differences over whom to appoint to key ministries.

Most significantly, the Defense Ministry remains vacant, more than a week after most members of the government were named. The new prime minister, Shiite Ibrahim al-Jaafari, has promised the powerful job will go to a Sunni, but no candidate acceptable to Shiites, Sunnis and Kurds has so far been named.

The appointment is seen as vital to the government's chances of addressing the spiraling violence by convincing moderate Sunnis that their future lies with Iraq's fledgling new democracy and not with armed rebellion.

As the violence rages and the government bickers, sectarian tensions have been building between the majority Shiites who dominate the government and the Sunnis marginalized from the political process by their refusal to participate in large numbers in the election.

The clash in Kufa came after a defiant speech by al-Sadr that was read at Friday prayers, and it brought reminders of the chaos al-Sadr's supporters caused last year when they revolted against U.S. troops in Najaf and elsewhere, including the Baghdad slum of Sadr City.

The sermon said the Iraqi government had done nothing to win the release of al-Sadr's followers in U.S. facilities or to stop raids on his offices, and warned that he might again mobilize his Mahdi Army militia.

"We dropped our weapons, but our hands are still on the trigger," al-Sadr said. "We have been patient and quiet with the truce, which [the U.S.] violated more than once. Consider the past period a training period for us, psychologically and morally."

Accounts of what caused the clash in Kufa varied, but there was agreement that Iraqi police or army officials shot and wounded some worshippers emerging from prayers.

A spokesman for the Defense Ministry said that guards for Aws al Khafaji, the imam who delivered al-Sadr's speech, were armed. Police questioned the guards, which led to a dispute that ended with an exchange of gunfire, the spokesman said.

Meanwhile, a mass grave was discovered northeast of Baghdad, near Sadr City.

Police said early yesterday that they recovered the bodies of 14 men who were buried in a garbage dump. Each victim had been blindfolded, handcuffed and shot once in the head.

The Muslim Scholars Association, a Sunni clerical group with ties to the insurgency, claimed that the victims were all farmers, members of the prominent Dulaimi clan, who had been abducted from a market the previous day by Iraqi army and police officers.

The association issued a statement naming all the victims. The men, brothers and cousins all between the ages of 25 and 40, had traveled to the Baghdad market to sell their goods, said Abdul Salaam al-Qubaisi, a spokesman for the association. They came from Madain, a town south of Baghdad that last month was the focus of reputed kidnappings that pitted Sunnis against Shiites.

The discovery of the mass grave raised further fears that bloody ethnic clashes are escalating.

On a bridge in Tikrit, Saddam Hussein's hometown, a car bomb killed seven and wounded three near a police checkpoint, the Iraqi Interior Ministry said.

In Suwayrah, about 25 miles south of Baghdad in an area known as "The Triangle of Death," a car bomb exploded in a market, killing 16 and wounding 36, according to the ministry.

Also yesterday, the Arab satellite news station Al-Jazeera broadcast two videos of kidnap victims. The videos contained ultimatums for their countries to either withdraw troops or stop doing business in Iraq.

In one video, men wearing masks brandished rifles at Australian engineer Douglas Wood, 63. Al-Jazeera reported that kidnappers had given Australia 72 hours to withdraw its 300 troops from Iraq. A banner visible at the left of the screen read al-Mujahedeen Shura Council.

Another video showed six employees of a Jordanian company called Jaafar Ibn Mansour being held by a group called al-Baraa bin Malik. Two men pointed AK-47s at the employees, also pictured holding their passports. Al-Jazeera said the kidnappers were demanding that all Jordanian companies cease business in Iraq.

More than 200 foreigners have been kidnapped in Iraq since Saddam was ousted in April 2003. Some have been seized for ransom; others have been used to pursue political goals. More than 30 have been killed by their captors.

In the face of the increasingly grisly news, the U.S. military claimed significant progress in breaking the organization of Abu Musab al-Zarqawi, the most wanted terrorist in Iraq.

It released a statement yesterday that highlighted the killing and capture of 20 top lieutenants in al-Zarqawi's network in recent months and included excerpts of demoralized testimonies from several detained bombmakers, drivers, propagandists and terror-cell leaders.

The statement also described the near-capture of al-Zarqawi in a Feb. 20 raid between Hit and Haditha.

Al-Zarqawi's driver, Abu Usama, recounted that "Zarqawi became hysterical" as coalition forces closed in on his vehicle, according to the statement. The statement said al-Zarqawi grabbed an American-made rifle and U.S. dollars and escaped, leaving behind his computer, pistols and ammunition.

Information from the Chicago Tribune and The Associated Press is included in this report.