"These are challenging times and we have to guard against such attempts by using force against extremists and militants on the one hand and exhibiting wisdom and patience on the other," said the president.

Witnesses said that firing was reported from different areas of Karachi after the attack and the police also used tear gas shells to disperse the protesters.

Television pictures showed a big cloud of smoke over the scene and television reporters said angry worshippers attacked media vehicles and police mobiles.

The Shia march was taken out of Nishtar Park and was on way to head to the final destination. The organizers decided to continue procession despite the blast. They appealed to the mourners to be peaceful.

People gather at the site where a bomb blast struck a major Shiite Muslim procession in Karachi, Pakistan, Monday, Dec. 28, 2009. [Xinhua]

It is the third bomb blast in Karachi in three days. Scores of peoples were injured in the two blasts on Saturday and Sunday, according to the police.

Karachi Mayor Mustafa Kamal asked the citizens to remain peaceful and do not show any reaction. He expressed shock over attacks on property and vehicles after the blast. He said that the blast was aimed at destroying peace of the city.

During the first Muslim month of Muharram, Shiites across the world mourn the death of the Prophet Mohammad's grandson Imam Hussain in the Iraqi city of Kerbala in the year 680.

Pakistan's security forces have been on high alert as Muslims are marking the holy day of Ashura, the Shia calendar's biggest event.

On Sunday evening at least 15 people, including mourners and policemen, were killed and over 100 injured when a suicide bomber ripped through a Muharram procession near a Shia mosque in the Pakistan-controlled Kashmir.

Pakistani volunteers stand near a dead body of a person killed by a bomb blast which struck a Shiite Muslim procession in Karachi, Pakistan, Monday, Dec. 28, 2009. [Xinhua]