The BBC's Aleem Maqbool says the attack was followed by rioting
At least 30 people have been killed and dozens injured in a suicide bombing on a Shia Muslim march in the Pakistani city of Karachi, officials say.
The attacker had been walking amidst a procession with tens of thousands of people, said the interior minister.
After the explosion, marchers turned their anger on ambulance workers, security forces and journalists.
Pakistan's security forces have been on high alert as Shia Muslims marked the holy month of Muharram.
Ashura on Monday was the climax of the holy period, commemorating the death of the Prophet Muhammad's grandson.
There was no immediate claim of responsibility for the attack, which comes amid an upsurge of violence by Taliban militants in Pakistan.
Karachi has a long history of sectarian violence between Shias and Sunnis.
There have been numerous attacks on such processions across the country over the last few days, says the BBC's Aleem Maqbool.
On Sunday, eight people were killed when a suicide bomber targeted a Shia march in Pakistan-administered Kashmir.
Interior Minister Rehman Malik blamed Monday's blast on extremists who wanted to destabilise Pakistan.
"Whoever has done this, he cannot be a Muslim. He is worse than an infidel," he told reporters.
Karachi police chief Waseem Ahmed said the severed head of the bomber had been found, reports Reuters news agency.
One survivor, Naseem Raza, told AP news agency: "I saw walls stained with blood and splashed with human flesh. I saw bloodstained people lying here and there."
Fleeing the scene of the blast, another mourner told AFP news agency: "My sister, her husband and children are dead."
The bombing unleashed further pandemonium as angry Shia mourners fired shots in the air.
Rioters torched dozens of shops and vehicles, while members of the security forces who had been guarding the procession were pelted with stones.
TV footage showed smashed police vehicles and ambulances.
Karachi Mayor Mustafa Kamal appealed for calm.
"I am hearing people are clashing with police and doctors. Please do not do that," AP news agency quoted him as saying.
"That is what terrorists are aiming at. They want to see this city again on fire."
Our correspondent says an incident like this was feared by the authorities. Stringent security measures had been put in place across the country over the last month.
Hundreds of civilians have been killed in bomb attacks in recent months as Pakistan's army pursues an offensive against Taliban militants in South Waziristan and surrounding areas.
Pakistan also has a long history of violence between Sunni and Shia Muslims that is estimated to have killed several thousand people in the last three decades alone.
Some radicals in the Sunni majority regard Shias - who make up about 20% of the population - as heretics.
Where you in the procession in Karachi? What is your reaction to the bombing? Please send us your comments using the form below. A selection of your comments may be published, displaying your name and location unless you state otherwise in the box below.
The BBC may edit your comments and not all emails will be published. Your comments may be published on any BBC media worldwide.