Voting began in Pakistan after an election campaign overshadowed by concerns of manipulation and violence.
Nearly 106 million people are registered to vote for members of the lower house of parliament and four provincial assemblies.
The PTI party of former star cricketer Imran Khan is hoping to beat ex-leader Nawaz Sharif’s PML-N.
But the Human Rights Commission says there have been “blatant” attempts to manipulate the polls.
Hundreds of thousands of troops will be deployed to secure the ballot, which opened at 08:00 (03:00 GMT). Votes will be cast for 272 National Assembly seats that are directly contestable.
Pakistan is no stranger to political turmoil and the last few months have proved no exception. The man who won the last election is watching this contest from prison, after a scandal stemming from the Panama Papers led to Mr Sharif being jailed for corruption.
In an interview, the former prime minister’s daughter Maryam Nawaz who was arrested last month with her father on related charges criticised the all-powerful military.
“When a prime minister refuses to put down his head and do their [the military’s] bidding, they pull him down with four things; get a religious fatwa issued against him, call him a traitor, call him a friend of India, or call him corrupt. They use these things against every elected prime minister,” she said.
In the eastern city of Lahore, the first person to enter a polling station was female business executive Maryum Arif, who told AFP she planned to vote for the incumbent PML-N as it had “served Pakistan.”
Women in Pakistan have the right to vote though many are unable to exercise this right, with many who live in socially conservative areas pressured into not voting.
Authorities are trying to change this by saying that at least 10% of voters in each constituency must be women in order for the results to be valid.
What’s the context?
Pakistan has been ruled on and off by the military during its 71-year history. This election is significant because it will mark only the second time that one civilian government has handed power to another after serving a full term.
But the run-up to the vote has been controversial.
The Pakistan Muslim League-Nawaz (PML-N) complains of a targeted crackdown by the powerful security establishment, with the alleged help of the courts, in favour of Imran Khan and his Pakistan Tehreek-e-Insaf (PTI).
On Sunday, a judge in the High Court of Islamabad appeared to back up that allegation, saying that the military Inter-Services Intelligence (ISI) organisation had been interfering in the judiciary.
Several PML-N candidates also say they have been coerced to switch to the PTI, and nearly 17,000 party members are facing criminal cases over breaking unspecified election rules. The Pakistani military denies interfering in politics.
Independent media, meanwhile, say there have been blatant attempts to muzzle them. There are also serious concerns about the participation of internationally designated militants in the election process.
Has Imran Khan’s time come?
As the early voters queue up outside polling stations, the question on everyone’s mind will be whether Mr Khan will be the next prime minister.
He entered politics in the late 1990s when memories of Pakistan’s cricketers winning the World Cup were still fresh. But only in 2013 did his party emerge as a serious contender, with its leader touted as a harbinger of change out to fight corruption.
He lost by a wide margin, but has continued to lead what many see as a divisive campaign.
This election is the closest he’s ever got to being prime minister. Will that happen? Or will the PML-N stay the largest party after a sympathy wave by voters for the man he helped oust?
Presentational grey line
Who are the main candidates?
The election is generally seen as a contest between the PML-N and the PTI.
Nawaz Sharif a 68-year-old three-time PM was convicted by an anti-corruption court and sentenced in absentia to 10 years in prison. He was disqualified from office last year by the Supreme Court after the Panama Papers leak revealed his family’s ownership of several luxury flats in London.
Sharif returned from London earlier this month with his daughter, Maryam, and both were arrested. Their appeal hearings will not be held until after the vote.
Sharif’s brother, the former governor of Punjab province, Shehbaz Sharif, 66, is now leading the party.
But Imran Khan and his PTI have been rising in the polls. Mr Khan, 65, told last week that his opponents will “lose because of their track record” when in power.
The country’s best-known opposition politician, who is running on an anti-corruption ticket, has denied colluding with the military. Analysts say he will have to make serious inroads in Punjab province a PML-N stronghold in order to win the vote.>