He also cited “attacks on media workers” and “restrictions on access to the internet and telecommunication services” as reasons to worry about “allegations of interference” in the process.
Many analysts have said this is among Pakistan’s least credible elections.
Voters in Lahore told the BBC that the internet blackout on polling day meant it was not possible to book taxis to go and vote, while others said they could not co-ordinate when to head to polling stations with their family members.
An interior ministry spokesman said the blackouts were necessary for security reasons.
Support from the military in Pakistan is seen as important to succeed politically, and analysts believe Mr Sharif and his party currently have their backing, despite their differences in the past.
Maya Tudor, associate professor at the University of Oxford’s Blavatnik School of Government, said the lead taken by Imran Khan’s PTI was “shocking” in the context of the country’s past.
“A win would be remarkable – in every single other election in Pakistan’s recent history, the military’s preferred candidate has won,” Dr Tudor explained.
As many as 128 million people were registered to cast their votes, almost half of whom were under the age of 35. More than 5,000 candidates – of whom just 313 are women – contested 266 directly-elected seats in the 336-member National Assembly.
Pakistan’s former ambassador to the United States, Maleeha Lodhi, said Pakistan “desperately” needs political stability to address what she described as “the worst economic crisis in its history”.
But, in a hopeful note, Ms Lodhi said Pakistan’s voter numbers show a “belief in the democratic process”. (Source: BBC)