The networks were closed from 10:00 am (0500 GMT) in Karachi and Quetta, gripped by political and sectarian violence between militants from the majority Sunni Muslims and minority Shiite communities.
"We had intelligence reports about (the) possibility of a very big terrorist attack in Karachi and it was because of this reason that the mobile phone service was shut down," senior police official Javed Odho told AFP.
"It is a preventive measure taken to effectively counter terrorism threat," he said, adding that the service will remain suspended until Friday evening.
Odho said the decision was taken after top police officials held lengthy meetings to deal with the threats. "Pros outweighed cons. This was the only effective way to block conversation of terrorists, who use mobile phones to plan attacks."
A senior official of the Pakistan Telecommunications Authority (PTA) confirmed suspension of the mobile phone services in Karachi and Quetta, the capital of insurgency plagued Baluchistan province.
Authorities feared that mobile telephones could be used to coordinate attacks or trigger a remote-controlled bomb.
Police in Karachi arrested four suspected militants on Tuesday who were planning a wave of sectarian attacks during Muharram in the city, where 40 people were killed during three days of bloody attacks.
The four belonged to Lashkar-e-Jhangvi, a banned Sunni militant group blamed for many deadly attacks on Shiites.
Odho said thousands of policemen have been deployed all over Karachi to perform duties alongside paramilitary rangers.
He said the service would be restored Friday evening but warned of a serious security threat in the province especially during the first 10 days of Muharram.
"At least eight provincial districts have been declared sensitive and the army troops will be on call to deal with any untoward situation," Durrani said.
Pakistan says 35,000 people, including more than 3,000 soldiers, have been killed as a result of terrorism since the 9/11 attacks and the 2001 US-led invasion of neighbouring Afghanistan.
Police in central Punjab province arrested two suspected terrorists from the eastern city of Gujrat who had been planning attacks during Muharram, provincial police chief, Haji Habib-ur Rehman told a news conference in Lahore on Friday.
Police detained Shafiq Ahmed, 45, and his son Abdul Rehman, 17, who had links with banned militant outfit Tehreek-e-Taliban Pakistan. Five suicide vests, hand grenades and Kalashnikov rifles were recovered in a raid on their house.
They were accused of involvement in a number of terrorist activities, including an attack at a Pakistan Air Force base in Kamra in August this year, Rehman said.
During interrogation they confessed "they had plans to launch attacks on security forces and religious gatherings of minority Shiite community", he said.
Rehman said security has been beefed up in Punjab with deployment of around 118,000 police troops. The army will be on standby and can be called if needed, he said.
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