Early unofficial results from the May 11 vote suggest his Pakistan Muslim League-Nawaz (PML-N) and Imran Khan's Pakistan Tehreek-e Insaf (PTI) received strong support.
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Our correspondent Abubakar Siddique (@Sid_Abu) on the situation in Karachi.
Pakistan's Landmark Elections Marred By Irregularities In Karachi
Allegations of vote rigging, violence and boycott in Pakistan's biggest city Karachi have overshadowed today's parliamentary elections.
Early in the day, a bomb attack on the office of secular Awami National Party killed at least 11 people and many more were injured.
Many polling stations across the city of 21 million people opened late and political parties began complaining about irregularities almost instantly.
In the early afternoon, Jammat-e Islami -- Pakistan's oldest Islamist political party -- boycotted the election in Karachi and the nearby city of Hyderabad. It accused the Muttahida Qaumi Movement (MQM), the political group that dominates the two cities of engaging in massive rigging.
The two parties have a long history of rivalry because the MQM overturned Jammat-e Islami's control over Karachi and has kept the sea port city as a stronghold since late 1980s.
Later in the day two more Islamist parties -- the Sunni Ittehad Council and Jamiat Ulma-e Pakistan -- as well as an ethnic faction the MQM boycotted the polls.
For its part the MQM also announced to pull out of the contests in Liyari, a Karachi neighborhood known as a traditional stronghold of President Asif Ali Zardari's Pakistan Peoples Party (PPP).
Late in the day, the Election Commission of Pakistan acknowledged problems in Karachi. It extended polling by three hours in many of the city's constituencies and announced that it would postpone voting in 42 other polling stations of National Assembly district 250, where most of the irregularities took place.
The election commission has not said when it will hold the polling again, but there will either be a new election in the whole constituency, or the figures of the re-polling on the 42 stations will be added to the overall tally.