Cellphones didn’t just arrive in Pakistan. But someone could be fooled into thinking otherwise, considering the tens of millions of Pakistanis pouring into mobile phone stores these days.

In one of the world’s largest — and fastest — efforts to collect biometric information, Pakistan has ordered cellphone users to verify their identities through fingerprints for a national database being compiled to curb terrorism. If they don’t, their service will be shut off, an unthinkable option for many after a dozen years of explosive growth in cellphone usage here.

Prompted by concerns about a proliferation of illegal and untraceable SIM cards, the directive is the most visible step so far in Pakistan’s efforts to restore law and order after Taliban militants killed 150 students and teachers at a school in December. Officials said the six terrorists who stormed the school in Peshawar were using cellphones registered to one woman who had no obvious connection to the attackers.

But the effort to match one person to each cellphone number involves a jaw-dropping amount of work. At the start of this year, there were 103 million SIM cards in Pakistan — roughly the number of the adult population — that officials were not sure were valid or properly registered. And mobile companies have until April 15 to verify the owners of all of the cards, which are tiny chips in cellphones that carry a subscriber’s personal security and identity information.

In the past six weeks, 53 million SIMs belonging to 38 million residents have been verified through biometric screening, officials said.

“Once the verification of each and every SIM is done, coupled with blocking unverified SIMs, the terrorists will no longer have this tool,” said a senior Interior Ministry official, who was not authorized to speak publicly about the government’s security policy. “The government knows that it’s an arduous job, both for the cellular companies and their customers, but this has to be done as a national duty.”

As Pakistan’s decade-long struggle against Islamist extremism has stretched on, residents have grown accustomed to hassles such as long security lines and police checkpoints. Now they must add the inconvenience of rushing into a retail store to keep their phones on.

“I spend all day working and sometimes have to work till late in the night. . . . I cannot afford to stand in line for hours to have my SIM verified,” said Abid Ali Shah, 50, a taxi driver who was waiting to be fingerprinted at a cellphone store. “But if I don’t do it, my phone is my only source of communication that I have to remain in touch with my family.”

Though Pakistan’s first cellphone company launched in 1991, there was only sparse usage until the turn of the 21st century. Since then, the number of cellphone subscribers has grown from about 5 million in 2003 to about 136 million today, according to the Pakistan Telecommunications Authority.

The mobile phone subscription rate now stands at about 73 percent, roughly equal to the rate in neighboring India, according to the World Bank. It’s even common for Pakistanis in remote or mountainous areas, where electricity can be sporadic and few have access to vehicles, to own a cellphone.

With 50 million more SIM cards left to be verified, phone companies are dispatching outreach teams deep into the countryside and mountains to notify customers of the policy.

“It’s a massive, nationwide exercise with a tight deadline, but hopefully we will be able to verify our customers by the April deadline,” said Omar Manzur, an executive at Mobilink, which has 38 million customers in Pakistan. “We have sent out 700 mobile vans all across Pakistan to reach out to these far-flung areas, the villages and small towns.”

One region that appears largely unaffected by the plan is the immediate area around the Pakistan-Afghanistan border, where many Islamist militants have historically sought refuge. Pakistani cellphone networks generally do not provide service to those areas, and residents try to get coverage from Afghan networks, officials said.

Cellphone owners’ fingerprints are being matched with those on file in a national database the government began creating in 2005. Those whose prints are not in the database must first submit them to the National Database & Registration Authority. Some residents, including several million Afghan refugees not eligible for citizenship, also have to obtain a court affidavit attesting they will properly use their cellphones.

Over the years, several countries, including South Africa and India, have implemented broad systems for obtaining and storing residents’ biometric information. But analysts and communications experts say they can’t recall a country trying to gather biometrics as rapidly as Pakistan is doing.

“In a country like this, where the infrastructure is not available in many areas, this looks unprecedented,” said Wahaj us Siraj, the chief executive officer of Nayatel, a major Pakistani Internet supplier.

Once the nationwide verification process is complete, police and intelligence officials will have a much easier time tracing the origins of crimes or terrorist attacks, said Ammar Jaffri, the former deputy director of Pakistan’s Federal Investigation Agency.

Jaffri noted that cellphones have often been used to detonate explosive devices in Pakistan. Authorities are also struggling to curb extortion carried out by criminals, often affiliated with banned militant groups, who make threatening phone calls demanding money.

Jaffri said Pakistanis should just accept that a SIM card “becomes part of you” and that any privacy concerns do not override government regulation of airwaves.

“We have new technology now, and we shouldn’t be afraid of these things, we should face it,” said Jaffri, president of the Pakistan Information Security Association. “Watching people when they move, it’s natural: Every country does it. ”

As they show up at cellphone stores, some Pakistanis are learning firsthand just how lax Pakistan had been in tracking SIM cards.

At a Mobilink office in Islamabad, Muhammad Safdar, 30, was told that six different SIM cards were attached to his name.

“I think some of my friends had my ID card number,” Safdar said. “Earlier it was very easy to simply redeem that number and get a SIM issued in that name.”

Ghulam Rasool, a 24-year-old Afghan citizen living here, waited in line only to learn that the SIM card he had bought at a fruit market four years ago was now illegal.

“Before, no one asked, but now they are, and it has to be in my name,” said Rasool, who emerged from the Mobilink office with a new phone number. “Everyone has my old number, and now I have to contact hundreds of people” in both Pakistan and Afghanistan.

Still, many Pakistanis are taking the process in stride, saying they are willing to do whatever it takes to reduce terrorism. They are skeptical, however, that this will be the answer to ending a war that has killed more than 50,000 Pakistani residents and soldiers over the past 13 years.

“If this can bring peace, it’s okay,” said Khan Gul, his thumb still stained with blue ink. “But I am wondering how a mobile phone verification can bring peace.”