Army chief General Ashfaq Parvez Kayani and Chief Election Commissioner Fakhruddin G Ebrahim were among those who exercised their right to franchise shortly after polling began at 8 am.
Footage on television showed Kayani walking into a polling station in Rawalpindi to vote.
Long queues were seen outside thousands of polling stations across the country despite fears of attacks by the Taliban, which had said that it would carry out attacks, including suicide bombings, on election day as it considers the polls part of an "infidel system" of democracy.
Over 100 people, including candidates, were killed in gun and bomb attacks by the Taliban and other militant groups during the campaign period.
Threats and attacks by the banned Tehrik-e-Taliban Pakistan forced three major parties -- Pakistan Peoples Party, Awami National Party and Muttahida Qaumi Movement -- to dramatically curtail their campaign.
More than 86 million people are registered to vote in the polls though elections in Pakistan have traditionally registered low turnouts.
Only 44 per cent of the electorate voted in the last general election in 2008.
A total of 4,670 candidates are standing for parliamentary elections while nearly 11,000 are running for the four provincial assemblies.
Parties like the Pakistan Tehrik-e-Insaf, Pakistan Peoples Party and Pakistan Muslim League - Nawaz have sought to attract many youths who will be voting for the first time.
Polling will continue till 5 pm and provisional results are expected to come in by early Sunday.
Tens of thousands of security personnel, including 70,000 soldiers, have been deployed across the country to maintain law and order during the polls.
The PML-N, led by former premier Nawaz Sharif, is widely tipped to emerge as the single largest party.
A spirited campaign by cricketer-turned-politician Imran Khan helped his Pakistan Tehrik-e-Insaf make inroads in the most populous province of Punjab and in urban areas.
The outgoing government led by the PPP was the first one in Pakistan's history to complete its full term of five years.
In the past, governments have been ousted by the army, which has ruled Pakistan for more than half of its history.
Image: Election workers sit in a van with ballot boxes, polling booths and polling material before transporting them to polling stations in Rawalpindi | Photograph: Mian Khursheed/Reuters
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