Two of the blasts in Karachi targeted the Awami National Party office, killing 11 and wounding 36, said Naeem Shah, a spokesman for Karachi police.
A third explosion was in the Karachi suburb of Landhi. It killed three people and wounded nine, according to the deputy commissioner of Malir district, Qazi Jan Mohammed. Voting continued afterward, he said.
A fourth explosion, which wounded four people, was in the Peerabad area of Karachi, Shah said.
Fakhruddin Ebrahim, the chief election commissioner for Pakistan, said he contacted the military over security concerns in Karachi and the harassment of polling station staff in the city. He said polling material was stolen in several areas.
Eleven people were killed in two separate bombings in Pakistan's volatile Balochistan province, district police officer Allauddin Kasi said. One of the attacks was on a vehicle carrying voters who had cast their ballots, he said.
Another targeted independent candidate Khadim Shah, the prime minister's office said in a statement condemning the bombing.
Elsewhere in Balochistan, at least four people were killed and eight injured in a clash between two groups at a polling station, police official Sardar Muhammad said.
In another incident, 12 Awami National Party supporters were hurt by hand grenade thrown at a party electoral office in Quetta, said Syed Mobeen Ahmed, a deputy inspector general of police.
Brig. Muhammad Abdur Raheem, the military spokesman for Balochistan, said polling still went well in the province apart from interruptions caused by a few incidents. There was a good turnout, including by women voters, he said.
In the northwestern city of Peshawar, 12 people, children among them, were wounded when a bomb exploded at a polling station in a school, said Habibullah Arif, a local deputy commissioner.
Of the 86 million voters registered to cast ballots, there were 36 million new voters, according to the Election Commission of Pakistan.
More than twice as many candidates are women than was the case five years ago, with 161 running compared with the 64 who contested the 2008 poll, according to the United Nations
As the nation makes the transition after years of mostly military rulers, the economic, political and security situation remains unstable.
As well as high inflation and poverty rates, Pakistan has seen outbreaks of violence, in some cases by Islamic extremists.
Since April, the Taliban in Pakistan have killed dozens in attacks on the three main political parties. Many urban voters and parties regard resurgent fundamentalism as one of country's biggest threats.
More than 600,000 security personnel were deployed nationwide leading up to the election, Information Minister Arif Nizami said Friday.
Pakistan's army, which helped deliver 650 tons of ballots to polling stations, deployed 91,000 troops around the country, a military spokesman said.
The ruling Pakistan People's Party is led by Bilawal Bhutto Zardari, the son of assassinated former prime minister and party leader Benazir Bhutto.
While his party became the first civilian government to complete a full five-year term -- the three governments after the death in 1988 of military strongman Zia ul-Haq were all brought down by the army -- its legacy is a deeply fractured country with a faltering economy.
The party's main opposition came from Nawaz Sharif's Pakistan Muslim League-N. One of the country's leading industrialists and richest men, Sharif has been prime minister twice before and was overthrown in a coup when Gen. Pervez Musharraf seized power in 1999.
Viewed as a religious conservative, his party -- Pakistan's second largest -- believes it would have won elections in 2008 had the assassination of Bhutto not given a massive boost to the ruling party.
Another contender was Imran Khan, the former cricket star and heartthrob who leads the Tehreek-e-Insaf (Movement for Justice) party.
Not in contention is Musharraf, who returned in March from four years of self-imposed exile to take part in the elections. A court banned him from taking part in politics and his party, the All Pakistan Muslim League, announced a boycott.