A suicide bomber has killed at least 16 people at Shia processions in Pakistan.
Another 25 people were wounded, including children.
The blast ripped a hole in the walls of a Shia mosque in the city of Rawalpindi, near the capital Islamabad.
Hundreds of worshippers kept marching, even though other explosive devices were found at the site.
Pakistan's Taliban claimed responsibility for today's violence and said it would stage more attacks on Shias over the next few days.
The Rawalpindi incident occurred hours after two bomb blasts killed at least three people near a gathering of Shias in Karachi.
Hardline Sunni militant groups linked to al-Qaeda have in recent months stepped up attacks against Pakistan's minority Shias whom they regard as non-Muslims.
Pakistan's military has failed to break the back of a persistent insurgency despite launching several offensives against their strongholds in the northwest near the border with Afghanistan.
Pakistan is due to host a summit of eight developing nations this week.
Pakistan, Iran, Egypt, Turkey, Indonesia, Malaysia, Nigeria and Bangladesh are expected to discuss ways of boosting trade and investment at the event.
Pakistani Interior Minister Rehman Malik said the attacks were designed to hurt Pakistan's image and create the impression that its US-backed 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," he said.
Pakistani authorities are bracing for what could be a bloody weekend, the climax of the Shia mourning month of Muharram.
Muharram marks the anniversary of the Battle of Karbala where the grandson of the Prophet Mohammad and his family members were killed.
Meanwhile, Pakistan's Taliban movement threatened to attack Indian targets to avenge the country's execution of Mohammed Ajmal Kasab.
Mr Kasab was the lone survivor of the militant squad responsible for a rampage through Mumbai that killed 166 people in 2008.