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Anjum Naveed, Associated Press
A Pakistani protester hurls back a tear gas canister fired by police, not pictured, during clashes that erupted as protestors tried to approach the U.S. embassy, in Islamabad, Pakistan, Thursday, Sept. 20, 2012. Hundreds of Pakistanis angry at an anti-Islam film that denigrates the religion's prophet clashed with police in the Pakistani capital Thursday, the most violent show of anger in a day that saw smaller demonstrations in Indonesia, Iran and Afghanistan.

ISLAMABAD — Pakistan has blocked cell phone service in major cities to prevent militants from using phones to detonate bombs during a national day of protest against an anti-Islam film produced in the United States.

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An Interior Ministry official says the service is blocked in at least 15 cities, including Karachi, Lahore and Islamabad. The service is scheduled to be resumed at 6 p.m. local time.

The ministry official spoke on condition of anonymity because he was not authorized to talk to the media.

The government has declared Friday a national holiday and has encouraged people to peacefully protest a film that denigrates Islam's Prophet Muhammad.

The film has sparked violent protests throughout the Muslim world that have left at least 30 people dead, including two in Pakistan.