The son of assassinated prime minister Benazir Bhutto is also running
Tens of millions of Pakistanis are expected to vote today in an election that could have repercussions for the entire region.
If successful, the elections will mark only the second time a civilian government has handed power to another since the country's independence in 1947.
The race to be the country's next prime minister is tight, but most polls are predicting a narrow victory for the former Pakistan cricket captain Imran Khan.
The head of the centre-right Pakistan Tehrik-e-Insaf party is likely to have to form a coalition if he wants to govern though.
His main opponent, the three-time former prime minister Nawaz Sharif, was arrested on corruption charges when he landed in Lahore from London earlier this month.
He is now campaigning from prison in Islamabad and is reported to be suffering from health issues.
In his place as candidate for the centre-right Pakistan Muslim League is his brother Shahbaz Sharif.
The party have said they have been "victimised" but remain "resilient" and have seen "a spike in popular support".
Pakistani politician Imran Khan, chief of Pakistan Tehreek-e-Insaf party, addresses his supporters during an election campaign in Lahore, Pakistan | Image: K.M. Chaudary/AP/Press Association Images
The military has been accused of influencing the election in Imran Khan's favour by censoring news coverage.
Some believe it would suit the army to have a prime minister with only a thin majority, therefore giving them power behind-the-scenes.
A third candidate, Bilawal Bhutto Zadari, from the Pakistan People's Party, could be the kingmaker.
The 29-year-old politician comes from a dynasty of Pakistan leaders.
He is the son of former prime minister Benazir Bhutto, who was assassinated in 2007.
His father is Asif Ali Zadari, who served as the 11th president of Pakistan.
Whoever wins the election will need to seek an alliance with Bhutto's party or possibly a collection of independents.
Voting day is a public holiday in Pakistan. Some 3,459 candidates, representing more than 100 political parties, are competing for 272 seats in the national assembly.
Sixty seats are reserved for women while 10 have been kept aside for religious minorities including Hindus.
Nearly half a million police personnel will be deployed to protect polling stations and to ensure votes can cast their ballots freely and safely.