Vote-counting was ongoing Wednesday in a knife-edge Pakistan general election as former cricket hero Imran Khan sought power on a day marred by a bloody suicide bombing and allegations of military interference.

Analysts said the vote was too close to call immediately after polls closed at 6:00 pm (1300 GMT), despite widespread allegations of pre-poll meddling by the armed forces in favour of Khan, which saw the process dubbed Pakistan's "dirtiest election".

As election workers sorted through massive piles of paper ballots, several parties -- including the incumbent Pakistan Muslim League-Nawaz, Khan's main rival -- alleged the count was being manipulated.

"We have serious reservations ... The counting process has been carried out behind closed doors and our polling agents have been taken out," PML-N spokesman Marriyum Aurangzeb said in televised comments.

"Results of both the (provincial and national) assemblies have been withheld, we want an answer for that. What change are they going to bring behind closed doors?"

Her allegations were echoed by other parties including the Pakistan Peoples Party (PPP), which could become kingmaker in a coalition government.

In a WhatsApp message, the PML-N also claimed that security forces had taken over polling stations.

The Election Commission's secretary Babar Yaqoob flatly denied the allegations.

Gallup Pakistan estimated turnout at between 50 to 55 percent in an electorate of nearly 106 million, similar to the previous contest in 2013.

There were more than 19 million new potential voters, including millions of women and young people.

A suicide bombing in the southwestern city of Quetta, claimed by the Islamic State (IS) group, killed at least 31 people and cast a shadow over voting day. Local officials said when the bomber tried to enter a polling station police stopped him, and he blew himself up.

It was the second major attack by IS this month in Balochistan province after an earlier blast at a campaign rally killed 153.

- Two-way race -

The vote was a rare democratic transition in the populous but poor nuclear-armed Muslim country, which has been ruled by the powerful military for roughly half its history.

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

The PPP, headed by Bilawal Bhutto Zardari -- son of slain prime minister Benazir Bhutto -- could be called upon to form a coalition with any winner.

Election researcher Azeema Cheema said the contest remained "too competitive to call", with the most populous province of Punjab -- which has 141 out of the 272 directly elected seats in the National Assembly -- likely to be the key following fierce campaigning in battleground areas there.

To form a majority government outright, the contest's winner would need to snatch 137 seats. But analysts predict a coalition will be more likely.

Voters in Lahore, capital of Punjab and traditionally a PML-N stronghold, said they were abandoning the party in favour of PTI.

"I have voted for PML-N my whole life but this time I voted for PTI because Imran Khan has promised free education and health," said 75-year-old Uzma Akram.

Up to 800,000 police and troops were stationed at more than 85,000 polling stations across the country, after a string of attacks targeting political events in the final weeks of the campaign killed more than 180 people, including three candidates.

An earlier attack in Balochistan Wednesday left one policeman dead when a hand grenade was thrown at a polling station, while in the northwestern town of Swabi one PTI worker was killed in an exchange of fire with a rival party.

- 'New Pakistan' -

Khan, 65, campaigned on populist promises to build a "New Pakistan" and vowed to eradicate corruption, clean up the environment and construct an "Islamic welfare" state.

But the erstwhile playboy's bid for power was dogged by widespread accusations he is benefiting from the support of the country's powerful security establishment. The media, activists and think tanks have decried a "silent coup" by the generals.

The military has rejected the accusations.

Election authorities granted military officers broad powers inside polling centres which further stirred fears of manipulation. Journalists were barred from entering polling stations in Lahore and Rawalpindi at several points throughout the day.

Khan has also 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 says it is the target of the alleged military machinations. Nawaz Sharif was ousted from power last year and jailed over a corruption conviction days before the vote, removing Khan's most dangerous rival.

AFP