Heavy security is in place in Pakistan ahead of Thursday's annual day of mourning, Moharram, observed by Shia Muslims.
The day commemorates the martyrdom of Prophet Mohammad's grandson Hazrat Imam Hussain, killed in the battle of Karbala some 1,400 years ago.
Shia Muslims believe that Imam Hussain is the true leader of Islam following the Prophet's death, but Sunni Muslims disagree.
Militant groups from the two Islamic sects have often clashed in Pakistan.
Sectarian deaths
Security is especially tight this year following a spate of sectarian killings last month, after a hardline Sunni activist was executed for the murder of an Iranian diplomat.
Moharram marks the climax of 10 days of mourning for the Shia Muslims in Pakistan and elsewhere.
Crowds of Shia Muslim men march through the streets in an annual ritual of self-flagellation, beating themselves with bare hands and, sometimes, chains and whips.
In February, Sunni hardliner Haq Nawaz was hanged for the murder of an Iranian diplomat, a Shia Muslim.
Violent clashes followed his execution, which was ordered by a court but was supported by the military government.
More than two dozen people were killed in and around Lahore, the capital of Punjab province, including at least 10 who lost their lives in an armed attack on worshippers in a mosque.
Sectarian killings also took place in Hangu, a town in the North West Frontier Province.
Clamp-down
In the days leading up to Moharram, the authorities detained hundreds of activists from hardline Shia and Sunni groups.
Thousands of soldiers are patrolling the streets of Lahore, where the largest procession of mourners will follow a traditional route through the old part of the city at midnight.
Except for the Shia mourners and officials, all outlets leading to the route have been blocked and local residents have been asked to remain inside their houses.
Troops have also been placed on alert along the international border with India.