ISLAMABAD, Pakistan - A suicide bomber attacked a Shiite Muslim religious procession in Pakistan's largest city yesterday, killing at least 30 people, as U.S.-backed President Asif Ali Zardari warned of a "conspiracy" against the country's democracy.

The fourth attack on Pakistan's Shiite minority in recent days fueled fears that a sectarian conflict alongside the nuclear-armed country's battle with the Pakistani Taliban and other Sunni Muslim extremist groups could topple Zardari's fragile civilian government, elected last year after eight years of military-led rule.

On Sunday, a senior intelligence official with the U.S.-led International Security Assistance Force in Afghanistan said the Pakistani government "could fall on a given day." Under ISAF policy, the official spoke on condition of anonymity.

Karachi, where yesterday's attack took place, is Pakistan's economic capital and lifeline to the outside world, but it is also an ethnic tinderbox. Until three Shiite marches were targeted in recent days, however, it had been spared the extremist attacks that have ripped through Pakistan's other urban centers over the last two years.

Yesterday's bombing targeted thousands of Shiites marching to observe Ashoura, the most important day of a monthlong mourning period for the seventh-century death of the prophet Muhammad's grandson Imam Hussein.

"I fell down when the bomb went off with a big bang," said Naseem Raza, 26, who was in the procession. "I saw walls stained with blood and splashed with human flesh."

Residents in nearby apartments tossed down body parts that had been cast into their homes from the blast. Authorities found the intact head and torso of the suicide bomber on the third floor of a nearby office building, where it had crashed through a window, said bomb-disposal squad official Munir Sheikh.

The blast also wounded 60 people, said Sagheer Ahmad, health minister in Sindh province, where Karachi is located.

"This is a brutal and barbaric act of terrorism," said Farooq Sattar, a federal minister from Karachi. "The conspiracy is very clear: to derail the economic hub of Pakistan and destabilize the country."

Outraged Shiites responded to the attack by setting fire to buildings and vehicles at the blast site and pelting security forces with stones.

More than 500 people have been killed in attacks in Pakistan since mid-October, when the army launched a major anti-Taliban offensive in the country's northwest.

The Obama administration and its Western allies are growing anxious that political upheaval is imminent in Pakistan and that a collapse of Zardari's government could force the United States and its allies to choose between defending democracy and opting for whatever stability that outright or implicit military rule could provide in a crucial antiterrorism partner.

Members of Zardari's Pakistan Peoples Party fear that if push comes to shove, Washington would side with the military again, but neither weak civilian rule nor strong military rule would guarantee more effective Pakistani cooperation against Islamic extremists, one senior U.S. official said yesterday.

The officials in Islamabad and Washington who agreed to speak did so on condition of anonymity.

A Pakistani Supreme Court decision this month to strike down a political amnesty law and to order corruption cases reopened against Zardari and thousands of other politicians has undermined the government and strengthened the military's hand.

In a fiery speech Sunday at the graveside of his wife, assassinated former Prime Minister Benazir Bhutto, Zardari said forces he did not identify were plotting to topple his government. "They are not afraid of Asif Zardari. They are afraid of democracy," he said on the second anniversary of Bhutto's murder, generally thought to be committed by Islamic extremists.

Zardari underscored his message in a commentary yesterday in the Wall Street Journal, writing: "Some of these forces who were allied with dictatorship in the past now hope that the judicial process can undo the will of a democratic electorate and destabilize the country."

This article includes information from the Associated Press.