Security forces in the violence-plagued port city of Karachi have been put on high alert with the start of the Islamic month of Muharram to prevent any incidents of religious violence between the Sunni and Shiite hardline groups, police said.

Last month, unidentified gunmen killed at least 11 Shiites in two separate attacks.

On February 22, gunmen sprayed bullets outside a Shiite mosque, killing nine people, while five days later in another similar hit-and-run attack, two other Shiites were killed.

Police fear that extremist groups might resort to more attacks during Muharram when Shiites, who are a minority of in Sunni majority Pakistan, mourn the martyrdom of the grandson of the Prophet Mohammed (PBUH). Police and paramilitary Rangers will remain on alert all through the first 10 days, senior police officials say.

At least another 300 new security check points have been set up in the volatile neighbourhoods, including at Shiite mosques and roads through which the Muharram processions march, a senior police official said.

At several key places, video cameras have also been installed, he added. Police officials are also anticipating attacks by militants opposed to their government's cooperation to the U.S.-led war against terrorism.

On February 27, a lone gunman opened fire outside the U.S. consulate killing two policemen. After Saturday's arrest of Khalid Sheikh Mohammed there is a fear of retaliatory attacks by the Al Qaida and its sympathisers.