Other officials said security had been stepped up in major cities, with helicopters, hidden cameras and hundreds of intelligence personnel being deployed to prevent attacks.
In Quetta, Pakistan army also patrolled the streets as Lashkar-e-Jhangvi warned that it would attack Shia processions despite the killing of its two members a day earlier. "We will carry out more attacks on their (Shia) processions," Mohammed Zarar, a Lashkar-e-Jhangvi spokesman said in a telephone call to journalists at the Quetta Press Club. "Our mission has not ended with their martyrdom. We will continue the mission," he said.
At Muharram, Shia mourn the 7th century martyrdom of Hazrat Imam Hussein, the grandson of Prophet Muhammad (Peace be upon Him), and his followers in the Iraqi city of Karbala.
Large processions are organised on the 9th and 10th of Muharram - called Ashura - the climax of the Shia mourning rituals. This year, Ashura falls on Saturday and Sunday.
Sunni extremists consider the mourning rituals against the teachings of Islam and demand that Shia be declared non-Muslims.
Tariq Jameel, DIG Police Karachi, said Shia volunteers and security personnel would search people taking part in the processions. "The threat of terrorism is very much there and we are taking no chances," he told Reuters. "We have sealed every road and deployed security personnel on the roof-tops on the entire route of the processions."
More than 100 people were killed in two attacks on Shia during Muharram in Quetta in 2003 and 2004. There were also suicide bombings inside Shia mosques last year in Karachi and attacks in other major cities.
Lashkar-e-Jhangvi, blamed for most of the sectarian bloodletting by the police, has also targeted top government officials, Western concerns and religious minorities since Pakistan joined the US-led war on terror following the September 11, 2001 attacks on the United States.