PML-N leaders claimed that around 300 workers were arrested from Rawalpindi on Tuesday, while several hundred others had been picked up from Lahore and other cities.
The former prime minister and his daughter were in London, where Sharif's wife Begum Kulsoom is undergoing treatment for cancer.
They are set to be arrested after their arrival here from London following their conviction in a corruption case described by Pakistan's embattled former prime minister as a "blind revenge" amid a crackdown on his party activists.
Last week, Sharif was sentenced to a total of 11 years in prison and slapped with a £8-million fine (Rs1.3-billion) in the Avenfield properties reference case, while Maryam, his daughter, was sentenced to eight years with a £2-million (Rs335-million) fine.
Sharif's son-in-law Capt (retd) Mohammad Safdar, who was awarded one-year jail term, has already been arrested and shifted to Adiala prison.
Underscoring the tensions gripping Pakistan in the run-up to the poll, a suicide bomber had killed more than 100 people at an election rally a few hours earlier, in the deadliest such attack in the country in more than three years. Deputy Inspector General Operations Shahzad Akbar said 10,000 police officers will be deployed across Lahore on Friday to "maintain law and order". A second stint in power was ended by a military coup in 1999, prompting a period in jail for Sharif and years in exile in London. In one picture retweeted by Maryam, the father and daughter are seen bidding a teary farewell to Kulsoom Nawaz.
The military, which has ruled Pakistan for about half its history since 1947, has denied interfering in modern-day politics. It plans to place 371,000 soldiers around polling stations so there can a "free and fair" elections, an army spokesman said this week.
Addressing a press conference in London with Maryam by his side, Sharif had said he has chose to return to the country "despite seeing the bars of prison in front of my eyes". The former premier wasdisqualified from the top job by the Supreme Court last July, his third ousting since the 1990s. His brother Shehbaz became PML-N's president, but Sharif remains the power behind the throne.
Khan has also denied colluding with the military.
PML-N has also been riven by internal divisions.
The kind of reception Sharif receives on the streets of Lahore will be viewed carefully in Pakistan, where political popularity is often measured by the size of rallies that politicians can attract.
After boarding the flight Friday, Sharif posted a video on social media urging supporters to show up in droves for the July 25 election, and thereby resist what he has called a conspiracy to oust his ruling party from power.
"Those who think they can scare us.open your ears and hear this: we are winning this election", Shehbaz Sharif told reporters in Lahore on Thursday.