However, the run-up to the election has been marred by violence in which more than 100 people have been killed.
A bomb blast in the port city of Karachi on Saturday morning left 10 people dead and 32 others wounded, a hospital official said.
Tens of thousands of troops are deployed at polling stations after the Pakistani Taliban threatened to carry out suicide attacks.
Hours before polls opened, Pakistan sealed its borders with Iran and Afghanistan in a bid to keep foreign militants at bay.
Officials said the borders would remain closed for the next three days.
Queues started forming before polling stations opened at 08:00 (03:00 GMT) on Saturday.
At one polling station in the capital, Islamabad, more than 200 people waited patiently to vote.
At a polling station on the outskirts of the capital, BBC World Affairs correspondent Mike Wooldridge said around 1,000 of the 4,000 registered voters had cast their ballots by midday, including a large proportion of women.
EU observers in the eastern city of Lahore told the BBC that voting there was going smoothly and without any interruptions.
The BBC`s Saba Eitizaz in Peshawar reports long queues of women waiting to vote. Many are voting for the first time and are excited about being part of a historic change, our correspondent says.
Voting was delayed at some polling stations in Karachi because ballot boxes and papers had not arrived.
Polling stations will close at 17:00.
The Taliban on Friday warned voters to boycott polling stations in order to avoid attacks on the offices of political parties.
The militants have been blamed for numerous attacks throughout the campaign on Pakistan`s three most prominent liberal parties.
The Pakistan People`s Party (PPP) along with the Karachi-based Muttahida Quami Movement (MQM) and the Awami National Party (ANP) have been singled out for attacks by the Taliban.
As a result, the parties were forced to curtail their election campaigning.
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