HILLAH, Iraq — Iraq's insurgents showed yesterday that they still have terrifying capabilities, slipping a massive car bomb into an area normally off-limits to vehicles, then detonating it in front of a medical clinic where dozens of military and police recruits were lined up to undergo physicals, killing at least 115 people.

Shiite Muslim politicians said the attack was another attempt to provoke civil war between the Shiites, who will lead the country's first Iraqi-chosen post-Saddam government, and the largely Sunni Arab insurgency. They insisted they will not fight violence with violence — at least not yet.

The explosion in this Shiite city 60 miles south of Baghdad was the single deadliest insurgent attack since the fall of Saddam Hussein's government in April 2003. More than 100 others were wounded.

Many of the dead perished when buildings that housed government offices and shops in the busy downtown district collapsed from the force of the explosion. Body parts were scattered for blocks, and hysterical survivors trudged through pools of blood searching for relatives.

The blast came just one day after the Iraqi government announced the arrests of several key figures in the insurgency, including Saddam's half-brother, who's accused of funding the insurgency from Syria, and two men close to the Jordanian militant Abu Musab al-Zarqawi, who has claimed responsibility for many of the almost daily car bombings.

There was no information on how the Hillah bomber maneuvered the car into position in an area where vehicle traffic has been restricted for months. Local residents said the bomber would have had to pass through at least one police checkpoint before reaching the spot where the bomb was detonated.

Ahmad Shia'a al-Barrak, a former member of Iraq's Governing Council who is from Hillah, said many structures in the area were built in the 1930s and 1940s and were too fragile to withstand the massive shock.

"Many innocent people were killed and injured because the buildings collapsed," he said. "There are also some apartment buildings 100 meters from the explosion that were damaged. There is a cinema close by which was full of people. Luckily, the roof only shifted. Otherwise, the number of casualties would have been much higher."

The bombing comes at a time when the Sunni Arab insurgency is trying to disrupt the formation of a new government set to be led by majority Shiites for the first time in modern history.

"These terrorists want to spark an ethnic war inside the country, so they started to kill based on identity — for example, for being Shiite," said Dr. Saad Jawad, a senior official in the Supreme Council for the Islamic Revolution in Iraq, the country's largest Shiite political movement and a big winner in the Jan. 30 elections.

The Shiites have refrained from striking back, mostly at the behest of their most revered leader, Grand Ayatollah Ali al-Sistani, who is widely credited with bringing them this far. Al-Sistani wants nothing to impede the Shiites from gaining the political power they have craved in Iraq, and will not allow them to engage in a sectarian war.

It's not that they lack firepower. Although Iraqi security forces remain underequipped and ill-prepared to fight insurgents, nominally disbanded Shiite militias could easily field thousands of tough and effective fighters that could deal a crushing blow to the insurgency.

"We sacrificed a lot of blood. We have to be patient and not drift into a civil war as Ayatollah al-Sistani has said," observed Jalal Eddin al-Sagheer, a senior cleric and member of the Supreme Council.

Alliance leader Abdel Aziz al-Hakim recently hinted that the Shiites were waiting to take power before dealing with the insurgency. He indicated that a first step then would be to identify and purge the security services of any insurgency sympathizers.

"We must depend on the sons of the Iraqi people who believe in the new Iraq, and not on those bad elements that infiltrated the security circles and turned into a problem," al-Hakim told The Associated Press on Sunday. "We can't solve the security issue unless we reconsider the internal structure, to spot those bad elements."

No official figures are available, but an Associated Press count found that 234 people were killed and 429 people injured in at least 55 incidents from Jan. 1 until election day. Casualties rose in February, which saw at least 38 incidents that resulted in at least 311 deaths and 433 injuries.

Although the bombing did not appear to be an explicit attack against Shiites — the insurgents consider police and soldiers to be collaborators with American occupiers — most of the victims were Shiites.

In fact, insurgents have stepped up assaults against predominantly Shiite targets in recent weeks, most notably a series of suicide bombings and other attacks that left nearly 100 people dead over the two-day Ashoura commemoration that began on Feb. 18.

There was no claim of responsibility for yesterday's attack.