In the Shiite holy city of Karbala, 70 miles south of Baghdad, five large blasts went off shortly after 10 a.m. local time near two of Shiite Islam's most important shrines.
The explosions, thought to be the result of a suicide bomber, mortars and concealed bombs, killed at least 85 people and caused mass panic among religious pilgrims, thought to number in the range of 2 million.
A leading Shiite cleric placed the number of dead higher, at 112.
"The latest information we have is that there were 112 martyrs from the attack in Karbala and 235 wounded," Ahmed al-Safi said in Karbala.
About the same time, three explosions occurred at the al-Kadhimain shrine in Baghdad, killing at least 75 people. According to the U.S. military, three suicide bombers caused the explosions.
The attacks -- which resulted in the bloodiest day in Iraq since the fall of Saddam Hussein -- came as thousands of Shiite pilgrims celebrated Ashura, the most significant religious festival on the Shiite calendar.
During the ritual, which was banned under Saddam's Sunni regime, Shiite Muslims beat and cut themselves to pay tribute to Muslim saint Imam Hussein, who was martyred in 680 AD.
Several members of the Iraqi Governing Council blamed a Jordanian named Abu Masab Zarqawi for the attacks. U.S. forces believe Zarqawi is affiliated with Osama bin Laden's al-Qaida terrorist network, and have placed a $10 million bounty on his head. They intercepted a letter from Zarqawi last month urging suicide attacks on Shiite Muslims as a way of fomenting civil war in Iraq.
Shiite spiritual leader Ayatollah Ali al-Sistani blamed U.S.-led forces for Tuesday's attacks, saying they had not secured Iraq's borders.
"We put responsibility on the occupation forces for the noticeable procrastination in controlling the borders of Iraq and preventing infiltrators, and not strengthening Iraqi national forces and supplying them with the necessary equipment to their jobs."
Neither the U.S. military in charge of Baghdad nor the multinational force in Karbala had deployed troops close to the areas where thousands of Shiites gathered Tuesday.
Brig. Gen. Mark Kimmitt, deputy director of operations for the U.S. Army in Iraq, said it had been right to stay away from mosques to respect local sensitivities. He said there was good coordination with Iraqi security forces.
"We certainly don't believe that this indicates that the coalition is demonstrating weakness," Kimmitt said. "This was a clear and tragically well-organized act of terrorism."
Kimmitt also said curbing terrorism involves much more than simple border security.
"Terrorism cannot be stopped at the border, plain and simple," he said. "It can only be stopped through the width and breadth of the country, inside the government and the councils, inside the homes."
Also Tuesday morning, gunmen opened fire and threw grenades on a Shiite religious ceremony in Quetta, Pakistan, killing at least 41 worshippers and wounding more than 150 others. According to Quetta's mayor, Abdul Rahim Kakar, the men then blew themselves up. Kakar said authorities were questioning several suspects.
The attack in southwestern Pakistan came just hours after the coordinated series of incidents in Iraq, although there was no evidence they were connected.
Rioting spread through Quetta's streets, during which a Sunni Muslim mosque, shops and a television network office were set on fire.
Mayor Kakar told the Associated Press that he had immediately placed the city of 1.2 million under curfew, and had deployed troops and paramilitary forces to restore calm.
Over the last two decades in Pakistan, thousands have been killed in violence between Sunni and Shiite Muslims.