KARACHI, July 21 (Reuters) - Pakistani police defused a car bomb outside a shopping mall in Karachi overnight, forcing a bookshop to cancel a live telecast from London of the launch of the latest Harry Potter book, police said on Saturday.

Pakistan is in the grip of a terror scare, after Islamist militants launched an offensive, often using suicide attacks, that has so far killed 180 people this month.

The bookshop in Karachi's up-market Park Towers mall had hoped to attract young and old fans of the fictional boy wizard for the launch at 4 a.m. on Saturday (2300 GMT Friday).

Instead, police cordoned off the mall after receiving an anonymous tip-off by telephone.

Two boxes containing between 10 and 12 kg of RDX explosive were found inside a stolen Suzuki car.

"We defused both bombs. These were highly explosive RDX," Mohammad Iqbal, head of bomb disposal squad, said.

The Islamist militants' bombing campaign appeared to have been in revenge for the army's operation to crush a militant stronghold at a mosque in Islamabad earlier this month.

The government said at least 102 people were killed in the siege and assault on the Red Mosque, where a Taliban-style movement had taken root.

Most of the attacks have taken place in the northwest, and have targeted police and soldiers. Karachi had not been targeted hitherto, and police have not said so far whether they believe the discovered car bomb was related.

A suicide car bomb attack in the nearby town of Hub earlier this week targeted Chinese engineers. It killed 30 people, including several members of the Chinese workers' police escort. Police said it was linked to the Islamist backlash raging up north.

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