Officials say the death toll has risen to at least 32, with scores more wounded. Isis claimed in its Amaq news agency that the attack was carried out by a suicide bomber, but did not provide further detail or evidence for its claim.
A witness told The Independent it looked like the attack targeted a police van, as the incident underscored the threats to democracy in a country where terror attacks have diminished but not disappeared entirely in recent years.
"The explosion occurred near a school where polling was taking place," witness Majeed Akbar said. "There was a cloud of smoke after the blast, it filled the sky and then after that I saw a pool of blood, dead bodies and others injured."
Another witness, Abdul Haleem, was queueing up to cast his own ballot when he said he saw a motorcycle drive straight into a crowd of voters. Mr Haleem's uncle was killed in the blast that followed, he said.
"There was a deafening bang followed by thick cloud of smoke and dust and so much crying from the wounded people."
Authorities have yet to comment on the nature of the blast. A spokesman for Quetta hospital, Waseem Baig, said 15 dead bodies were brought in to his hospital alone and 33 injured were under treatment there, some in a serious condition.
Jaffer Kakar, a doctor, said two children and five policemen were among the dead.
Quetta, where the bombing took place, is the capital of Baluchistan province, which saw the worst of the pre-election violence.
Earlier this month, a suicide bomber killed 149 people at an election rally in the town of Mastung, also in Baluchistan province. The attack was claimed by Isis militants.
About 371,000 soldiers had been stationed at polling stations across the country to prevent attacks, nearly five times the number deployed at the last election in 2013.
Citing security concerns, the election commission announced that internet and mobile phone services in several districts in Baluchistan were suspended.
Election commission secretary Babar Yaqub told reporters that threats against polling stations, staff and even candidates have been received.
Hours earlier, militants lobbed grenades and opened fire at a military convoy escorting election staff and voting material in Baluchistan's district of Turbat, killing four troops.
Also on Wednesday, police said a shooting between supporters of two opposing political parties killed one person and wounded two in a village near the north-western city of Swabi.
Counting has now begun in the election which was seen as narrowly led in opinion polls by Imran Khan's PTI party, but otherwise too close to call.
Analysts see it as likely that PTI will be run close for the lead by the PML-N party of jailed former premier Nawaz Sharif. There is a good chance neither will take an absolute majority, opening the door to coalition talks and a kingmaker role for the PPP party, led by the son of former leader Benazir Bhutto.
Results are not expected until around 2am on Thursday morning (10pm Wednesday BST).
Additional reporting by agencies
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