A bomb has exploded near a Shi'ite religious procession in northwest Pakistan killing seven people, including four children.
Security forces are on high alert over fears of large-scale attacks on the minority sect across the country.
Pakistan has suspended phone coverage in many cities this weekend - an important one in the Shi'ite Muslim calendar - after a series of bomb attacks on Shi'ites triggered by mobile phones.
Hardline Sunnis have threatened more attacks as the Shi'ite mourning month of Muharram comes to a climax. More than a dozen people have already been killed this week observing Muharram, Reuters reports.
The attack on Saturday occurred in the city of Dera Ismail Khan, a stronghold of al-Qaeda-linked Sunni militant groups who regard Shi'ites as non-Muslims and have stepped up sectarian attacks in a bid to destabilise Pakistan.
In Karachi, more than 5000 police are expected to patrol the streets during Muharram events over the next two days, with hundreds more on alert.
Interior Minister Rehman Malik said more than 90% of bombs are detonated via phones. In some areas of the country, motorbikes carrying passengers have been banned due to fears that this is how attackers could be brought to their targets.
On Friday, mobile phone services were temporarily blocked in parts of the capital Islamabad, Karachi and Quetta.
Muharram marks the anniversary of the Battle of Karbala, where the grandson of the Prophet Mohammad and his family members were killed.
The schism between Sunnis and Shi'ites developed after the Prophet Muhammad died in 632 when his followers could not agree on a successor.
Islamabad hosting summit
A summit of eight developing nations began in Islamabad on Thursday.
Radio Australia reported Pakistan had hoped the summit would help revive its international standing, but a spate of killings on Wednesday cast a shadow over the gathering.
It is rare for Pakistan to host high-level international meetings because of its reputation.
Bangladesh, Egypt, Indonesia, Iran, Pakistan, Turkey, Malaysia and Nigeria were to discuss ways of boosting trade and investment.
Interior Minister Rehman Malik said the bombings in Rawalpindi and Karachi which killed 26 people on Wednesday were designed to hurt Pakistan's image and create the impression that its government is incapable of establishing stability.
"We are trying to build relationships, get investment in Pakistan and these groups are trying to derail the process."