At least 27 people died and dozens more were injured in northwestern Pakistan when a suicide bomber attacked a crowd of Shi'ite Muslims observing the religious festival of Ashura. Similar protests left more than 40 injured and two dead in Afghanistan.
The bomb blast sparked riots in Hangu, about 200 kilometers southwest of the capital.
Hundreds of Shi'ite protestors destroyed cars and burned shops before police and army reinforcements regained control of the city.
Interior Minister Aftab Sherpao says tensions in the city remain high but both Sunni and Shi'ite clerics are working to avoid another outbreak of violence.
"Religious leaders of both sects are appealing to the people to remain calm and not resort to any action which will create further complications in the area," he said.
He says a curfew has been imposed and checkpoints have been set up on the roads leading into the city.
Officials say security forces are on high alert throughout the country.
Ashura is the holiest day of the year for Shi'ite Muslims, who are a minority in Pakistan.
It marks the death of the Prophet Muhammad's grandson, which also led to the original schism between Sunni and Shi'ite Muslims.
Annual celebrations across the country are frequently targeted by Sunni extremists.
Sectarian violence also broke out in neighboring Afghanistan, where hundreds of Shi'ite and Sunni Muslims clashed, leaving dozens of people injured.
Witnesses say at least two mosques were destroyed during the fighting, which started outside Herat in western Afghanistan.