The Pakistani Taliban has claimed responsibility for killing 23 people on the eve of the D8 summit, in Pakistan's city of Rawalpindi, and have said that more would follow.
"People belonging to Shia community are infidel and blasphemous and we will not spare them. You will see further attacks in coming days," The Telegraph quoted said a Taliban spokesman, as saying.
Shi'ites have reported receiving threatening text messages.
The religious festival of Muharram, a 10-day period when processions mourn the death of the Prophet Mohammed's grandson, ends on Sunday and is a particular target of Sunni extremists. Terrorist groups frequently target Pakistan's minority Shia community.
According to human rights groups, more than 300 Shi'ites have been killed so far this year.
Witnesses to the attack in Rawalpindi said a suicide bomber threw a grenade into a procession on its way to a mosque, before detonating an explosive vest. A total of 23 people were killed and 62 injured, out of which eight were children, officials have said.
Earlier, a blast outside a Shia mosque in the southern city of Karachi killed at least two people, and gunmen in north-west Pakistan killed four police officers in an ambush. (ANI)