After an acrimonious campaign, polls opened in Pakistan on Wednesday to elect the country's third straight civilian government — a first for this majority Muslim nation, which has been directly or indirectly ruled by its military for most of its 71-year history.

Rights groups have warned that the rancorous campaign and widespread allegations of manipulation imperil the wobbly transition to democratic rule and raise the spectre of bitter post-election challenges of fraud.

Early voting was heavy in some polling stations in Islamabad, the capital, and in the Punjab provincial capital, with several political party leaders standing in line to cast their vote.

The unprecedented participation of radical religious groups, including those banned for terrorist links but resurrected and renamed, has also raised fears that the space for moderate thought may shrink further in Pakistan.

Attacks against minorities have increased in recent years. One candidate, Jibran Nasir, an independent from Pakistan's financial hub of Karachi, received death threats and even had a fatwa, or religious edict, issued against him after he refused to condemn Ahmadis, who are reviled by mainstream Muslims as heretics because they believe the Messiah promised in Islam arrived over a century ago.

In Pakistan it is a crime for an Ahmadi to call himself a Muslim after a 1974 constitutional amendment declared them non-Muslims.

"I am speaking for the millions of Pakistanis who are too afraid to confront religious bigotry," Nasir said in a telephone interview. He also said that no arrests have been made of those who threatened him.

The leading contenders in Wednesday's polling are former cricket star Imran Khan and his right-of-centre Pakistan Tehreek-e-Insaf Party, or PTI, and the right-of-centre Pakistan Muslim League, the party of disgraced Prime Minister Nawaz Sharif, who is in jail serving 10 years on corruption charges. His younger brother Shahbaz Sharif took control of the party. The third-largest party in the running is the left-leaning Pakistan People's Party, headed by Bilawal Bhutto, the son of late Prime Minister Benazir Bhutto, assassinated by Pakistan's Taliban militants, whom she had vowed to eradicate.

Election officials say more than 11,000 candidates are vying for 270 seats in Pakistan's law-making Lower House of Parliament and 577 seats in four provincial assemblies. The 85,307 polling stations opened at 8 a.m. and will continue for 10 hours, an hour longer than in the 2013 polls. Voting for two parliamentary seats and six seats in provincial assemblies has been postponed due to attacks on candidates or disqualifications.

There are 105.96 million eligible voters in Pakistan, with 59.22 million men and 46.73 million women.

Pakistan's election commission reminded candidates their elections will be nullified if the female voter turnout does not reach 10 per cent. The requirement was imposed after the 2013 elections, when several areas banned voting by women, mostly in Pakistan's religiously conservative northwest. Some candidates were elected without a single woman marking a ballot.

The commission issued its reminder Tuesday after veteran rights activist Tahira Abdullah said local jirgas, or councils, of elders from 60 areas of the country, representing 16 different constituencies, had signed agreements banning women from voting.

Results will trickle in after polling ends at 1300 GMT, with the outcome expected early Thursday, according to election officials.

Citing security concerns, the election commission announced that internet and cell phone services in several districts in southwestern Baluchistan province had been suspended. Some of the worst violence during campaigning occurred in Baluchistan, where earlier this month a suicide bomber devastated a political rally, killing 149 people, including the candidate, Siraj Raisani. Voting in that constituency has been suspended.

Election commission secretary Babar Yaqub told reporters late Tuesday that threats against polling stations, staff and even candidates have been received.

His statement came just hours after militants lobbed grenades and opened fire at a military convoy escorting election staffers and voting material in Baluchistan's district of Turbat, killing four troops. At the request of the election commission, Pakistan's military is deploying 350,000 troops countrywide outside and inside polling stations.

International and domestic election observers will monitor voting. The European Union Election Observation Mission has 120 monitors at polling stations in major centres across Pakistan, with the exception of Baluchistan, where a suicide bomber earlier this month killed 149 people, including a candidate.

Under Pakistani law, separate seats are reserved for women and for non-Muslim minorities, which comprise 4 per cent of the population.

Voter turnout in 2013 was 54.8 per cent.