Voting is under way across Pakistan in landmark national and provincial elections.
The vote marks Pakistan's first transition from one civilian government to another in its 66 year history.
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 11 people dead and 40 others wounded, officials 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 on Saturday at 8am.
At one polling station in the capital, Islamabad, more than 200 people waited patiently to vote.
A BBC correspondent at a polling station on the outskirts of the capital said around 1000 of the 4000 registered voters had cast their ballots by midday, including a large proportion of women
Around two hours after polling started, a bomb attack was reported in Karachi, apparently targeting an ANP candidate outside the party's political office.
Police said 11 people were killed and more than 40 others were wounded.
The local ANP candidate, Amanullah Mahsud, who was in the office at the time of the attack, was wounded but not seriously.
The BBC reports the attack happened in the Landhi area of Karachi, an industrial district known for Taliban activity.
Another ANP candidate and his son were shot dead close to the area last week.
Sporadic violence linked to the election was also reported in several other areas.
Seven people were hurt outside a polling station in Peshawar after a bomb attached to a motorcycle went off and an explosion was also reported in Quetta in the south-west.
In Chaman on the border with Afghanistan, one person died and three others were hurt after a clash between two groups at a polling station.
Polling stations will close at 5pm.