WASHINGTON – Authorities in Baghdad estimate that at least 800 Shiite pilgrims died yesterday in a stampede during a religious procession, allegedly caused by a rumor of a suicide bomber among the crowd.

Early reports from the scene suggested most of the fleeing worshippers were either trampled to death or drowned after falling from a bridge connecting the Sunni neighborhood of Azamiyah to the Shiite area of Kazimiyah across the Tigris River. It was the bloodiest day in Iraq since the fall of Saddam Hussein. Prime Minister al-Jafaari yesterday declared three days of mourning.

One eyewitness, an American adviser to Deputy Prime Minister Ahmad Chalabi, suggested yesterday that chemical weapons may have killed some of the pilgrims. “There are a couple of doctors that say it was consistent with some kind of poisoning,” Francis Brooke said yesterday. “Many of the deaths are not consistent with a stampede.” Rumors abounded in Baghdad yesterday, according to another source in the Iraqi capital, that some of the victims inhaled chemical gas.

That account did not square with the report of Iraq’s interior minister, Bayan Jabr. He said yesterday, “Pushing started when a rumor was spread by a terrorist who claimed that there was a person with an explosive belt, which caused panic. … Some fell from the bridge, others fell on the barricades.”

In the coming days autopsies will be performed on the fallen, which could shed light on whether victims were killed by a stampede or other causes.

America’s ambassador to Iraq, Zalmay Khalilzad, yesterday extended his sympathies to Mr. Jafaari. According to one administration official, he also promised to lend American expertise to Baghdad hospitals and morgues to help determine the cause of death.

Yesterday’s tragedy could exacerbate tensions between the Sunni Arab minority and the majority Shiite Arab population in Iraq. Shiites have been targeted by terror master Abu Musab al-Zarqawi, who, in a 2004 letter to Osama bin Laden, wrote that he sought to start an ethnic civil war in Iraq. Officially, Shiite leaders have urged their people to refrain from violent counter attacks against Sunni Arab communities. However, in recent months militias affiliated with radical cleric Muqtada al-Sadr have launched assassinations against alleged terrorists and other enemies.

The Reuters news agency quoted a leader of the Shiites’ Supreme Council for the Islamic Revolution in Iraq, Ammar al-Hakim, as saying, “We hold the terrorists, Saddamists, and radical extremists responsible for what happened.”

Some political leaders in Baghdad blamed the government for failing to take more protective measures. “Early security measures should have been taken to protect the lives of citizens and organize their processions,” the leader of Iraq’s Communist Party, Hameed Majid Mousa, told Al-Arabiya television, according to the Associated Press. “We all know that there are terrorists who lie in wait for such events and prepare to ambush the people. … Why are the processions not organized?” The Iraqi Communist Party supported the American intervention in Iraq and has joined the government.

“I understand that the Iraqi government is on the scene with an immediate response in terms of security and medical support,” a State Department spokesman, Sean McCormack, said yesterday. “All reports are that they’re doing a great job, and we certainly have every confidence that they’ll be able to handle the situation, but again, we stand ready to assist in any way possible.”

NATO’s secretary-general, Jaap de Hoop Scheffer, yesterday bluntly blamed the terrorists for the tragedy in Baghdad. “This tragedy was the direct result of terrorism. Hundreds of innocent people, mostly women and children, have died because of the fear and panic that terrorists are sowing in Iraq.”

The Associated Press quoted a barefoot, soaking-wet eyewitness yesterday as describing the panic: “We heard that a suicide attacker was among the crowd,” Fadhel Ali, 28, said. “Everybody was yelling, so I jumped from the bridge into the river, swam, and reached the bank. I saw women, children, and old men falling after me into the water.” Reports from the two-lane bridge said it was lined with discarded shoes and sandals. The pilgrims were commemorating the death of an 8th-century saint, Imam Moussa ibn al-Kadhim.