ISLAMABAD (Reuters) - Pakistan is suspending phone coverage in many cities this weekend, an important one in the Shi'ite Muslim calendar, after a series of bomb attacks on Shi'ites triggered by mobile phones.

Hardline Sunnis have threatened more attacks as the Shi'ite mourning month of Muharram comes to a climax. More than a dozen people have already been killed this week attending Muharram processions.

"All the blasts that occurred in the last 15 days were mobile phone-based," Interior Minister Rehman Malik told reporters on Friday.

Intelligence information indicates more attacks have been planned for the coming days in the capital city of Islamabad, Karachi and Quetta. Mobile phone service will be suspended for hours in the three cities and dozens of others over the weekend.

In Karachi, more than 5,000 police are expected to patrol the streets during Muharram events over the next two days, with hundreds more on alert.

"The army is on standby. If any untoward incident occurs, we will call them, if need be," Malik said.

Muharram marks the anniversary of the Battle of Karbala, where the grandson of the Prophet Mohammad and his family members were killed.

Pakistani intelligence officials say extremist groups led by Lashkar-e-Jhangvi have intensified their bombings and shootings of Shi'ites in the hope of triggering conflict that would pave the way for a Sunni theocracy in U.S.-allied Pakistan.

(Reporting by Aisha Chowdhry and Mehreen Zahra-Malik; Writing by Randy Fabi; Editing by Andrew Roche)