Major Pakistani telecom operators reported that internet connectivity was down in some regions on Friday, including major urban centres, with one company reporting that flooding after heavy rain was responsible for the problem.
The telecoms regulator said it was investigating.
State-owned Pakistan Telecommunication Company Ltd's (PTCL) optical fibre network was experiencing faults and users in northern and central regions were facing an outage, it said on Twitter.
"Due to heavy rains and floods, PTCL's optical fibre network is experiencing some technical faults," it said, adding it was working to restore services.
PTCL's problems had a knock-on effect on other service providers, including on cellular data.
Telenor Pakistan, backed by Norway's Telenor, said its internet network was down because of a network issue at its internet service provider.
Pakistani users posting on social media complained of other telecom providers also being down but there were no statements acknowledging problems from other companies.
Widespread internet blackouts were reported in the capital, Islamabad, and the eastern city of Lahore, Pakistan's second-largest urban centre, where fixed-line broadband users as well as cellular network users reported no connectivity.
But numerous users reported that Pakistan's largest telecommunications provider, Jazz, owned by Amsterdam-listed global provider Veon, was up and running.
"Jazz network is by and large unaffected with our robust architecture and multiple layers of protection to provide consistent experience to our users," Jazz's head of external communications, Khayyam Siddiqi, told Reuters.
He said the provider was experiencing a spike in data traffic following the outages on other networks.
Pakistan, a country of about 220 million people, has a large and growing internet user base.
According to the Pakistan Telecommunication Authority, there are 116 million users of 3G and 4G services and 119 million broadband subscribers.