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

But it has also been dubbed Pakistan's "dirtiest election" due to widespread accusations of pre-poll rigging by the armed forces who are believed to favour Khan.

Polling day violence struck when dozens of people were killed in the suicide bombing, claimed by the Islamic State (IS) group, in the southwestern city of Quetta.

It was the second major attack by IS this month in Balochistan province, after an earlier blast killed 153 people in Pakistan's deadliest ever suicide attack.

Hashim Ghilza, a local administration official, said the bomber attempted to enter the polling station.

"When police tried to stop him he blew himself up," he told AFP.

The blast left a scene of chaos all too familiar to residents of Quetta. Debris, bloodstains and charred vehicles littered the road outside the polling station as blood-spattered dead and injured were rushed to hospital accompanied by distraught loved ones.

- Two-way race -

The contest has largely become 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 its campaign.

Khan, 65, cast his vote in Bani Gala, a suburb of the capital Islamabad, telling the media it was "time to defeat parties which kept this country hostage for years".

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.

But other voters in Lahore, traditionaly 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 have been stationed at more than 85,000 polling stations across the country, with concerns for security 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 and three wounded when a hand grenade was thrown at a polling station in the village of Koshk.

In the northwestern town of Swabi one PTI worker was killed in an exchange of fire with a rival party, police said.

- 'New Pakistan' -

Analyst Rasool Bukhsh Rais said he did not think security fears would prevent Pakistanis from voting, adding that the Quetta blast "will not demoralise people".

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 benefiting 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 and said 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 manipulation.

Khan has also raised eyebrows in recent weeks by increasingly catering 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.

A third party, the Pakistan Peoples Party headed by Bilawal Bhutto Zardari -- son of slain prime minister Benazir Bhutto -- could be called upon to form a coalition with any winner.

Radical groups such as the Milli Muslim League, linked to Hafiz Saeed, the man accused of masterminding the 2008 Mumbai attacks, are also contesting the polls, though many are running under the banner of smaller, lesser-known parties.

More than 19 million new voters, including millions of women and young people, may prove decisive, analysts have said.