LAHORE, Pakistan Five people were injured in a bomb blast here Tuesday as a security blanket was thrown across Pakistan on the eve of the execution of a Sunni extremist who murdered Iranian diplomat, Sadeq Ganji.

AFP quoted the police as saying that the bomb exploded in one of this eastern city's most crowded markets, heightening fears of a violent sectarian backlash against the execution expected this morning.

No group has claimed responsibility for the blast, which coincides with a crackdown against Sunni radical group Sipah-i-Sahaba Pakistan (SSP) ahead of its member Haq Nawaz's execution.

Armored personnel carriers were patrolling the area around Mianwali Jail in central Punjab Province, where Nawaz was awaiting death for the murder of Iranian Cultural Center director Sadeq Ganji here in 1990.

Police were deployed in force around SSP-controlled mosques and Iranian representative offices throughout the country, according to Punjab Police Chief Asif Hayat.

He said raids on SSP hideouts and the detention of group members were continuing for the third day to "ensure the peace."

Police sources said some 875 Sunni fundamentalists had been detained in the crackdown, although the SSP claims more than 1,200 have been locked up without charge, including several leaders.

Tension was high at SSP headquarters in Jhang, Punjab, after police arrested Ziaur Rehman Jhangvi, the son of the group's slain founder Haq Nawaz Jhangvi.

In Rawalpindi, near the capital Islamabad, a group of SSP supporters burned tyres in the street and chanted slogans against the execution.

Chief SSP spokesman Maulana Mujeeb-ur-Rehman Inqalabi, who has gone underground to escape the police sweep, warned of violence if Nawaz is executed.

"We will agitate if the execution is not stopped," he said. "These detentions are an attempt to sabotage peace and create anarchy."

Hayat said police were simply "carrying out their legal obligations" to protect the public, and particularly the Shia minority, from further extremist attacks following a spate of recent sectarian murders.

"These detentions are in response to the killings of Shias in the last couple of weeks," he told AFP.

Thirteen people have been murdered in sectarian assassinations in Punjab and Sindh provinces since late January. Eight of the victims were Shias and five were Sunnis.

Military intelligence sources said the government had been warned of a violent backlash from Sunni extremists if the condemned man is executed.

Interior Minister Moinuddin Haider on Monday said all four provincial governments had been ordered to "take action against the religious extremists."

"Over the years many people have been killed in the name of sects and we cannot allow this to continue," Haider said.

Police and paramilitary troops have also been deployed at sensitive places in the Sindh capital Karachi, including the Iranian missions, city Police Chief Tariq Jamil said.

Hundreds of people have died, mostly in Punjab, in recent years in violence blamed on extremists from the rival sects, while traditionally good relations between Pakistan and Iran have become strained.