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.

The contest has largely boiled down to a two-way race between Khan's Pakistan Tehreek-e-Insaf (PTI), and the incumbent Pakistan Muslim League-Nawaz (PML-N) of ousted premier Nawaz Sharif, whose brother Shahbaz is leading the campaign.

The first voter to enter a polling station in the eastern city of Lahore was a woman, business executive Maryum Arif, who told AFP she planned to vote for the PML-N as "it has served Pakistan".

She was followed shortly after by Shahbaz Sharif, who called on Pakistanis to "get out of their homes and ... change the fate of Pakistan" before casting his own vote and flashing a victory sign.

Up to 800,000 police and military forces have been stationed at more than 85,000 polling stations across the country, with concerns for security after a string of bloody militant attacks in the final weeks of the campaign that have killed more than 180 people, including three candidates.

Early Wednesday one policeman was killed and three wounded in a hand grenade attack on a polling station in southwestern Balochistan, Pakistan's poorest and most restive province, local police there told AFP.

No group has yet claimed the attack, but Balochistan suffers from Islamist and separatists insurgencies and was the scene of several campaign bombings, including a devastating attack claimed by the Islamic State group which killed 153 people this month.

Security fears did not appear to deter Arif, the first voter at the polling station in Lahore, who told AFP that "the law and order situation is fine here". Heavy contingents of police and military could be seen ahead of polls opening.

In Bani Gala, a suburb of the capital Islamabad, media massed outside the polling station where Khan is due to cast his vote.

- 'Murky' -

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 the erstwhile playboy's 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.

Khan has also raised eyebrows in recent weeks by increasingly catered to hardline religious groups, particularly over the inflammatory issue of blasphemy, 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.

"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 young people, 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, as well as the attacks, which have fuelled concerns that Pakistan may be losing ground on hard-fought security gains in recent years.

mak-jaf-ga-st/mtp