The man behind what police called an "immature" bomb plot phoned police after having an apparent change of heart because of the many children at the site, said Karachi police investigation chief Manzoor Mughal.

"Police have foiled an attempt to blow up a car bomb by remote control outside the Pak Towers and defused 10 kilogrammes of explosives in a stolen car," he said. "Had it exploded it would have caused a huge loss of lives."

"There were several hundred people inside Pak Towers at the time we received the call from an unknown person that a bomb was about to explode," Mughal said.

The warning prompted the cancellation of the official launch of "Harry Potter and the Deathly Hallows" at the Liberty Books shop in the shopping centre in Pakistan's largest city.

Police were trying to trace the call, and were interviewing the woman from whom the car was stolen at gunpoint on July 10 for a description of the thief.

Amid global "Potter mania," around 2,000 Pakistani fans, mostly children and youths, had reserved copies of the book at the launch.

"At around 9:00 pm last night a call was received from an unknown caller," said Mughal.

"He said that he had put explosives in a car outside the Pak Towers, but then he realised it would cause damage to the children, and that is why he informed them."

A wave of deadly suicide bombings and other attacks has claimed more than 200 lives in a week in Pakistan in apparent backlash violence to the army's deadly storming of a pro-Taliban mosque in Islamabad earlier this month.