Footage broadcast by TV news channels showed voters queuing outside polling stations in the capital Islamabad as well as in Lahore, Karachi, Peshawar, Quetta and other cities. Voting will continue until 5 p.m. local time (1200GMT).
More than 128 million registered voters are eligible to cast their ballots following a nail-biting end to an intense election process that has been marred by terrorist attacks.
Ballots will be cast for 266 general seats of the lower house -- also known as the National Assembly -- and 749 general seats of the country’s four provincial assemblies.
Thousands of police and paramilitary troops have been deployed across the country to ensure peaceful polling against the backdrop of recent militant attacks on several polling stations in southwestern Balochistan and northwestern Khyber Pakhtunkhwa provinces.
The government also temporarily suspended mobile phone service across the country ahead of the polls following the terrorist attacks, the Interior Ministry said in a statement.
According to the Election Commission of Pakistan (ECP), as many as 28,626 candidates from about 150 political parties are vying for National Assembly and provincial assembly seats across the country.
Around 100 international observers are in Pakistan to monitor the crucial polls, according to the Information Ministry.
A fierce three-way contest is expected between Pakistan Muslim League-Nawaz led by three-time Prime Minister Nawaz Sharif, the center-left Pakistan People's Party headed by former Foreign Minister Bilawal Bhutto Zardari and independent candidates backed by the Pakistan Tehreek-e-Insaf party, whose founder and former Prime Minister Imran Khan is in jail.
He has been sentenced in three separate cases, including those involving corruption and leaking state secrets. The ECP has also barred his party from using its iconic cricket bat electoral symbol to identify candidates contesting the polls.
Several other regional and religio-political parties including Jamaat-e-Islami, Jamiat Ulema-e-Islam, the Muttahida Qaumi Movement, the Awami National Party and the Balochistan National Party are also competing for seats in the national and provincial assemblies.
A party requires 169 seats to form the central government with a simple majority.
Pakistan follows a parliamentary form of democracy where the lower house elects the prime minister, who himself must be a member of the National Assembly.
The four provincial assemblies elect their respective leaders, or chief ministers, in the same manner.