Lahore: A powerful blast ripped through a Shia procession in Pakistan's Punjab province on Sunday, killing at least 20 devotees and injuring more than 30 others, witnesses said.

The explosion targeted the procession marking the 'Chehlum' of Prophet Mohammad's grandson Hazrat Imam Husain in Khanpur, 350 km from Lahore, this afternoon.

Witnesses said people ran helter-skelter when the blast went off as the procession was making its way to the Darbar Husain 'imambargah' or prayer hall in the centre of the town.

A journalist at the site of the blast said that he had seen 15 bodies being brought to a hospital.

Officials said another five persons succumbed to their injuries at the same hospital.

Over 30 injured people were taken to hospitals in Khanpur and nearby Rahim Yar Khan, officials said.

"I was present some 300 metres from the site of explosion, which appeared to have been caused by a planted device," a witness named Abbas Raza said.

The impact of the blast was so powerful that he was thrown to the ground and could not recover his senses for several minutes, he said.

"Several people were lying in a pool of blood," Raza said.

Local residents took the injured to a nearby hospital before rescue teams reached the site, he said.

The dramatic footage showed the blast going off as the procession made its way along a narrow street with residential buildings on both sides.

Men, women and children ran in panic as the explosion threw up a dense cloud of dust.

Members of the procession were seen crying and shouting slogans against the police and attackers after the blast.

An angry crowd staged a protest outside the main police station in Khanpur. Police used teargas to disperse the protestors.

Hospital officials said many of the injured were hit by pellets. They also had injuries usually associated with bomb blasts, the officials said.

District police chief Sohail Zafar claimed only 13 people had died in the incident.

He did not rule out the possibility that a planted explosive device was used in the attack.

Earlier, regional police chief Zahir Abid Qadri had claimed that an 'alam' or flag carried by the procession had touched overhead electric wires, causing a transformer to explode.

Tension between Shias and activists of the radical group Sipah-e-Sahaba Pakistan and Lashkar-e-Jhangvi have mounted since LeJ chief Ishaq Malik was set free from prison last year.

"The involvement of the SSP or LeJ in today's incident cannot be ruled out," said a security official who did not want to be named.

Punjab Law Minister Rana Sanaullah ordered an inquiry into the incident. He said all law enforcement agencies had been asked to further tighten security.

The 'Chehlum' or the 40th day from Muharram was observed across Pakistan on Sunday.

Authorities put in place strict security arrangements as Shia processions have been attacked on this day in past years.

(Agencies)