The group of about 100 Shias was passing through the city of Dera Ismail Khan on their way to join a larger crowd marking the day when the bomb went off, said police official Rashid Khan, who reported the casualty toll. 

No one claimed responsibility, but the suspicion fell on Pakistani Taliban, who often carry out such attacks. 

On Sunday, Shias in Pakistan will celebrate Ashoura, which commemorates the 7th century death of Imam Hussein, the Prophet Muhammad's grandson. The Sunni-Shia schism over Muhammad's rightful heir dates back to that time. 

President Asif Ali Zardari condemned the bombing, which took place near the South Waziristan tribal region. 

"Such nefarious acts cannot deter the people and the government in their fight against the scourge of militancy," he said in a statement. 

Sunni extremists frequently attack Shias, who they consider heretics, and the Ashoura ceremonies are a prime targets, bringing out crowds of Shias to march in processions mourning Hussein. 

On Wednesday night, a Taliban suicide bomber struck a Shia Muslim procession in Rawalpindi, a garrison city near Islamabad, killing 17 people. Initially police said 23 people were killed in this attack, but later authorities put the death toll from bombing to 17. They provided no explanation for discrepancy in death toll. 

Also Wednesday, the Taliban set off two bombs within minutes outside a Shia mosque in the southern city of Karachi, killing one person and wounding 15 others. Pakistani Taliban have claimed responsibility for the attacks in Rawalpindi and Karachi. 

Qamar Abbas Zaidi, the spokesman for main Shia political party, Tehrik-e-Nifaz-e-Fiqh Jafariya, vowed this week to go ahead with Ashoura processions across the country despite threats of attacks. 

Pakistani authorities have deployed thousands of additional police across the country to beef up security for such processions, and authorities have suspended mobile phone service in all the major cities for two days to prevent such bombings. Officials say Taliban frequently use cellular phones as remote detonators for bomb attacks.