Pakistan government has banned more than 300 Ulema from leaving their home districts to prevent them from inciting violence when holy month of Muharram starts from Sunday.
Ulema consider processions in Muharram to be against the spirit of Islam.
On Friday, the government issued a list of 341 Ulema, who would be confined to their home districts for 90 days.
"There are reasons to believe that they will make speeches or indulge in activities prejudicial to the public safety," the order said.
Police and paramilitary rangers have also been put on high alert, specially outside mosques to prevent any attacks.
President Pervez Musharraf has banned several extremist groups in an attempt to curb violence, but police say extremists continue to operate in small cells, many with ties to Afghanistan's former Taleban regime and Osama bin Laden's al Qaeda.
In Pakistan's restive port city of Karachi, the capital of southern Sindh province, additional police and paramilitary troops were being deployed, provincial government spokesman Salahuddin Haider told Reuters.
"There is a possibility of reaction from extremist religious elements as the government is trying to root out extremism from the society," he said. "We can not afford to take any chances".