The voting was set to end at 5 pm (local time) but sizable number of voters kept arriving at the polling stations.
Subsequently, the Election Commission announced that voting will continue till 6 pm (6:30 IST).
In some constituencies in Karachi, polling was extended till 7 pm (7:30 IST) as voters faced problems earlier in the day.
Election officials said that turn out was expected to be 60 per cent.
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 the 342-member National Assembly while nearly 11,000 are running for the four provincial assemblies in Khyber Pakhtunkhwa, Punjab, Sindh and Baluchistan.
