Smaller political parties could play a key role in government formation, which would require 169 seats in the 336-member National Assembly. Voters directly elect 266 members while there are 70 reserved seats – 60 for women and 10 for non-Muslims – allocated according to the number of seats won by each party.
Independents, many of whom are supported by Khan, are free to join any party if they win, which could change fortunes after the vote. Khan has said that his candidates will not support either Sharif or Bhutto Zardari.
Elections are also held under the shadow of increasing terrorist attacks. On the eve of the election, 26 people were killed in two blasts at election offices in the south-western province of Balochistan.
The country is on high alert and the army is deployed at polling stations. Thousands of soldiers and paramilitary forces have been deployed on duty across the country. Pakistan also said it was closing its borders with Iran and Afghanistan for security purposes.