A suicide bomber has killed at least 31 people near a polling centre as Pakistanis voted in a knife-edge general election pitting cricket hero Imran Khan against the party of jailed ex-Prime Minister Nawaz Sharif.
The Islamic State militant group claimed responsibility for the attack on Wednesday in the western city of Quetta, where security sources say the bomber drove his motorcycle into a police vehicle.
A hospital spokesman said 31 people were killed and 35 others wounded in the attack that a Reuters witness said occurred near a voting centre in Quetta, the capital of Baluchistan province.
Earlier this month, a suicide bomber killed 149 people at an election rally in the town of Mastung in Baluchistan province. That attack was also claimed by Islamic State militants.
About 371,000 soldiers have been stationed at polling stations across the country to prevent attacks, nearly five times the number deployed at the last election in 2013.
According to the latest opinion polls, neither Khan nor Sharif is likely to win a clear majority in the election.
Khan has emerged as a slight favourite in national opinion polls but the divisive race is likely to come down to Punjab, the country's most populous province, where Sharif's party has clung to its lead in recent surveys.
The election has been plagued by allegations the powerful armed forces have been trying to tilt the race in Khan's favour after falling out with the outgoing ruling party of Sharif, who was jailed on corruption charges this month.
About 106 million people are registered to vote. The polls closed at 11.30pm AEST with observers declaring the election too close to call.
Results will start trickling in within hours and the likely winner should be known on Thursday morning.
Whichever party wins, it will face a mounting and urgent in-tray, from a brewing economic crisis to worsening relations with on-off ally the United States to deepening cross-country water shortages.
An anti-corruption crusader, Khan, 65, has promised an "Islamic welfare state" and cast his campaign as a battle to topple a predatory political elite hindering development in the impoverished mostly Muslim nation of 208 million people.
Khan has staunchly denied allegations by Sharif's Pakistan Muslim League-Nawaz (PML-N) party that he is getting help from the military, which has ruled Pakistan for about half its history and still sets key security and foreign policy in the nuclear-armed nation.
Such a delay could further imperil Pakistan's economy, with a looming currency crisis expected to force the new government to turn to the International Monetary Fund for Pakistan's second bailout since 2013.
Sharif's PML-N has sought to turn the vote into a referendum on Pakistan's democracy and campaigned to protect the "sanctity of the vote".
The Pakistan Peoples Party, which has been overtaken by Khan's PTI as the main challenger to PML-N but is widely seen as the likely kingmaker, has also alleged intimidation by spy agencies.
