Pakistan's airports have been placed on highest alert, with the threat of a terror attack imminent, officials say.
Mr Sharif was exiled from Pakistan after a bloodless coup in 1999
The government has also banned gatherings of five or more people near Rawalpindi's international airport.
The moves come days before exiled former Prime Minister Nawaz Sharif, who has a residence in Rawalpindi, is expected to return to the country.
Meanwhile, Lebanese and Saudi officials have urged Mr Sharif to honour a deal not to return to Pakistan until 2010.
Mr Sharif, leader of the opposition Pakistan Muslim League Nawaz (PML-N) party, insisted he would return home on Monday despite the Arab leaders' appeals.
The Federal Secretary of the Ministry of the Interior, Syed Kamal Shah, told the BBC that Pakistan's airports had been alerted to the terror warning, but did not elaborate on the nature of the threat.
Mr Sharif is expected to fly into the Rawalpindi airport on Monday from London, after being exiled from the country following a bloodless coup in 1999 by President Pervez Musharraf.
Hundreds of Sharif supporters have been detained across the country ahead of his return.
Saudi Arabia's intelligence chief Prince Muqrin bin Abdul Aziz and Lebanese MP Saad Hariri called on Mr Sharif to postpone his return after meeting Gen Musharraf.
In 2000, Mr Sharif was exiled to Saudi Arabia after being sentenced to life in jail on charges of hijacking and terrorism.
Mr Hariri's father - assassinated former Lebanese Prime Minister Rafiq Hariri - helped negotiate the Saudi deal, under which Mr Sharif was to remain in exile for 10 years.
But Saad Hariri was unable to convince Mr Sharif to stay out of Pakistan when the two met on Friday.
MUSHARRAF UNDER PRESSURE
9 March: Musharraf suspends chief justice for "abuse of power". Lawyers protest
April: Protests grow, amid clashes with police
12 May: 34 people die as rival political groups clash in Karachi
11 July: 102 people die when army storms radical Red Mosque in Islamabad
July-Aug: Sharp rise in suicide attacks by pro-Taleban militants
20 July: Supreme Court reinstates chief justice
9 Aug: Musharraf rejects emergency rule
23 Aug: Supreme Court says exiled ex-PM Nawaz Sharif can return
"We are trying to convince him not to do so," Mr Hariri said.
"I can assure you that Saudi Arabia truly cares about Pakistan and its security and the agreement should be honoured."
Mr Sharif has denied the deal was made and insists he will return with his brother, Shahbaz, also a politician.
"I will go back to Pakistan on 10 September with my brother because my country needs me," he said at a news conference in London.
Gen Musharraf's faltering government has continued to use the Arab connection in a last bid to prevent Mr Sharif's potentially explosive political comeback, says the BBC's Shoaib Hasan in Islamabad.
But the country's supreme court ruled in August that no law could prevent Mr Sharif's return.