Thursday's poll came amid surge in militant violence with 26 people killed in two explosions on Wednesday. Islamic State later claimed responsibility.
The election is also being held at a time of a deep economic crisis and in a highly polarised political environment, and many analysts believe no clear winner may emerge.
Thousands of troops have been deployed at polling stations across the country and borders with Iran and Afghanistan were temporarily closed to facilitate a peaceful election.
"As a result of the recent incidents of terrorism in the country precious lives have been lost, security measures are essential to maintain the law and order situation and deal with possible threats," the interior ministry said in a message on the social media platform X.
The move to suspend mobile networks sparked criticism from leaders of opposition parties, with the Pakistan Peoples Party's Bilawal Bhutto Zardari, the 35-year-old son of former premier Benazir Bhutto, calling for its "immediate restoration".
Chief Election Commissioner Sikandar Sultan Raja said the decision on mobile networks was made by law and order agencies following Wednesday's violence and the commission would not interfere.
Analysts believe a clear winner may not emerge from Pakistan's national election. (AP PHOTO)
Jailed former prime minister Imran Khan's Pakistan Tehreek-e-Insaf (PTI) party, in a post on X, called on people to remove passwords from their personal Wifi accounts "so anyone in the vicinity can have access to internet on this extremely important day".
Khan cast his vote via postal ballot from a prison in Rawalpindi on Thursday morning, his party's media team said.
Despite tightened security, one person was killed when gunmen opened fire on a security forces vehicle in the northwestern area of Tank, a source in the intelligence services said.
Grenade attacks were also reported in different parts of Balochistan's Kech district but polling remained unaffected since there were no casualties, Saeed Ahmed Umrani, commissioner of the Makran division, told Reuters.
Unofficial first results in the election are expected a few hours after voting closes and a clearer picture is likely to emerge early on Friday.
The main contests are expected to be between candidates backed by Khan, whose party won the last national election, and the Pakistan Muslim League of three-time premier Nawaz Sharif, who is considered the front-runner.
Bhutto Zardari has also run an aggressive campaign in an outside bid for the top office.
In spite of the bitter winter cold, long queues began forming at polling stations hours before voting was due to start.
"The country is at stake, why should I come late?" said 86-year-old Mumtaz, a housewife a decade older than Pakistan itself as she lined up in Islamabad.
Analysts say there may be no clear winner but the powerful generals could play a role. The military has dominated the nuclear-armed country either directly or indirectly in its 76 years of independence but for several years it has maintained it does not interfere in politics.
If the election does not result in a clear majority for anyone, tackling multiple challenges will be tricky - foremost seeking a new bailout program from the International Monetary Fund after the current one expires in March.
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