"PML-N and PPP were running under constraints and PTI was operating with complete freedom and establishment backing".
Election authorities have not yet confirmed when they expect to announce the results.
With 30 percent of the total vote counted, Khan's Pakistan Tehreek-i-Insaf or PTI, or Pakistan Movement for Justice, was listed by the ECP as leading in 113 of 272 contested National Assembly constituencies.
According to AFP, Gallup Pakistan estimated voter turnout at between 50 to 55 per cent in an electorate of almost 106 million - similar to the previous contest in 2013.
In addition to the bombing in Quetta, polling across the country opened to violence.
There were more than 19 million new potential voters, including millions of women and young people.
Claiming that tactics were being used by the electoral body, Rabbani asserted that "After 7pm polling agents were made to exit the stations".
"This is complete chaos", analyst Azeema Cheema told AFP.
Debris, bloodstains and charred vehicles littered the road outside the polling station as the dead and injured were rushed to hospital accompanied by distraught loved ones.
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Pakistan's military has ruled for approximately half the period since the country's independence in 1947, staging coups three times.
Pakistan cricket hero turned politician Imran Khan claimed victory Thursday in the country's tense general election, following accusations of "blatant" vote-rigging by rival parties.
In other areas, it said, "polling staff appeared to be biased toward a certain party", without naming the party. Its spokesman, Nadeem Qasim, said Khan had violated constitutional provisions on "the secrecy of the ballot" and that his vote could be disqualified.
As voters crowded the polling station in the National Assembly elections, the bomber blew himself up, killing at least 31 people and wounding at least 35.
Pakistan's mainstream political parties have rejected the elections results terming the Wednesday's polls "rigged".
Also Wednesday, shooting between supporters of two opposing political parties killed one person and wounded two in a village near the northwestern city of Swabi.
An anti-corruption crusader, Mr Khan has promised an "Islamic welfare state" and cast his populist campaign as a battle to topple a predatory political elite hindering development in the impoverished mostly Muslim nation of 208 million people, where the illiteracy rate is more than 40 per cent. Journalists were barred from entering polling stations in Lahore and Rawalpindi at several points throughout the day. The provincial results show that the PML-N was leading on 129 seats in Punjab followed by PTI with 121 in the provincial assembly of 299 members.
The PML-N says it is the target of the alleged military machinations.
Former premier Sharif, a vocal critic of the army, was sentenced to 10 years in prison on corruption charges in absentia and was arrested after returning to Pakistan on July 13.
Sharif, the younger brother of disgraced ex-prime minister Nawaz Sharif, took over the ruling Pakistan Muslim League past year after his brother was found guilty of corruption.