The attack was the third in as many days in Pakistan's most populous city, a major South Asian port that has emerged as a significant logistics hub for supply trucks headed to Afghanistan in support of U.S. and NATO-led forces. 'Whoever has done this is not a Muslim and worse than an infidel,' said Interior Minister Rehman Malik, who blamed the attack on extremists trying to destabilize Pakistan. The blast hit as the government finds itself under pressure on multiple fronts. The United States is pushing Islamabad to do more to root out Islamic extremism along the nation's mountainous border with Afghanistan and beyond. The unpopular administration of President Asif Ali Zardari is also under growing political pressure from rivals who accuse it of corruption. The public also is increasingly weary after several months of bombings by Taliban militants in retaliation for a government offensive against their strongholds. Officials said the bomb in Karachi exploded on Mohammed Ali Jinnah Road as a huge throng was passing for Ashoura -- a major event on the Shiite calendar -- despite the presence of thousands of security forces already on high alert. Immediately after the attack, angry worshippers lashed out at journalists and police and set fire to shops and vehicles, according to local media reports. Television footage showed a large cloud of black smoke over the area. 'A black-bearded man in his 40s inserted himself into the main procession and a few seconds later a huge blast occurred,' one eyewitness, who was not identified, told a local network. Local police chief Waseem Ahmad appealed for calm, adding that the charred head of the suspected bomber had been located. Shiite Muslims are a minority in Pakistan and their open religious displays are periodically attacked by militants from the majority Sunni Muslim community. Ashoura, known by Shiites Muslims as 'the day of grief,' takes place on the 10th day of Muharram, the first month of the Islamic calendar. The day commemorates the death of Imam Hussein, a grandson of the Prophet Muhammad. Even as the United States has urged Pakistan to crack down on militants operating along its shared border with Afghanistan, Islamabad is reluctant to target the Taliban too aggressively. This stems in part because Islamabad sees the Taliban as a way to counter the influence in Afghanistan of its traditional adversary, India. Khan, a special correspondent, reported from Islamabad, and Magnier reported from New Delhi. 