The first voter to enter a polling station in the eastern city of Lahore was a woman, an AFP reporter there saw. More than nine million new female voters have registered for the vote in the deeply patriarchal country.

The vote is meant to be a rare democratic transition of power in the nuclear-armed country which has been ruled by the powerful military for roughly half its history.

But it has been dubbed Pakistan's "dirtiest election" due to widespread accusations of pre-poll rigging by the armed forces, with Khan believed to be the beneficiary.

Up to 800,000 police and military forces have been stationed at more than 85,000 polling stations across the country ahead of the poll.

In Bani Gala, a suburb of the capital Islamabad, media massed outside the polling station where Khan is due to cast his vote; while in Lahore heavy contingents of police and military could be seen ahead of polls opening.

Security fears did not appear to deter citizens in Lahore a day before the election, however.

"These so-called security threats are an excuse to scare the voters," shopkeeper Kashif Ahmed told AFP.

The election has largely boiled down to a contest between former prime minister Nawaz Sharif's incumbent Pakistan Muslim League-Nawaz (PML-N) and Khan's Pakistan Tehreek-e-Insaf (PTI).

Khan is campaigning on populist promises to build a "New Pakistan", vowing to eradicate corruption, clean up the environment and construct an "Islamic welfare" state.

But his campaign has been dogged by widespread accusations he is benefitting from the support of the country's powerful security establishment, with the media, activists and think tanks decrying a "silent coup" by the generals.

The military has rejected the accusations, saying it has no "direct role" in the electoral process.

Election authorities have granted military officers broad powers inside polling centres that have further stirred fears of possible manipulation.

The erstwhile playboy Khan has also raised eyebrows in recent weeks as he has increasingly catered to hardline religious groups, sparking fears a win for PTI could embolden Islamist extremists.

The PML-N, on the other hand, says it is the target of the alleged military machinations, with candidates under pressure and Nawaz Sharif ousted from power last year and jailed over a corruption conviction days before the vote, removing Khan's most dangerous foe from the race.

His brother Shahbaz is leading the party's campaign.

"Our predictions are very murky right now," Bilal Gilani, executive director of pollster Gallup Pakistan, told AFP on Tuesday.

More than 19 million new voters, including millions of women and youth, may prove decisive in the close race.

Gilani said many remain undecided: "It's still up for grabs."

The campaign season has also been marred by the expansion of far-right religious parties and a string of bloody militant attacks that have killed more than 180 people, including three candidates.

The attacks have fuelled concerns that Pakistan may be losing ground on hard-fought security gains in recent years.