Court orders arrest of ex-PM’s brother and supporters detained in run-up to return.
“Politically, they are very scared of a big show of popularity upon his arrival,” Iqbal said.
A provincial police official said 250 “trouble makers” had been picked up.
Before his departure from London, Sharif said: “Thousands of people have been arrested but by the grace of God our supporters are in the millions. That is not going to deter us.”
Sharif is expected to arrive in Islamabad at around noon (0700 GMT) on Monday.
“Security is at high alert and tomorrow visitors won’t be allowed in, only people with confirmed tickets,” a security official told the Reuters news agency.
“Musharraf is shouting slogans of war against terror in order to win US support”
Ali Abu Hamza, Chichawatni, Pakistan
Sohail Rahman, Al Jazeera’s correspondent in Islamabad, said there were indications Sharif would take a Middle Eastern carrier and arrive in Pakistan through Muscat.
Sharif has to take the detour since he remains banned from using Pakistani state airlines.
After landing in Islamabad, Sharif plans to lead a procession to his political power base in the city of Lahore, 300km away.
Kamal Hyder, Al Jazeera’s correspondent in the capital of Punjab province, said security forces were preventing Sharif’s supporters entering of leaving the city.
Muqrin bin Abdul Aziz, Saudi Arabia’s intelligence chief, has previously called on Sharif to honour the terms of a Saudi-brokered deal which sent him into exile seven years ago.
“Nawaz Sharif should respect his commitment to the most revered Muslim country [Saudi Arabia] and its leadership and complete 10 years in exile,” Muhammad Ali Durrani, Pakistan’s information minister, said.
Sharif was sentenced to life in prison on treason charges but released on condition that he live in exile for 10 years.
There are suggestions that on arrival in Pakistan both he and his brother, Shahbaz, could be rearrested on corruption charges.
Media reports have said a “VIP cell” at a 16th-century fortress is being prepared for Sharif.
A court in the eastern city of Lahore issued an arrest warrant on Friday for Shahbaz, who is also returning from exile, in connection with a murder case.
The Supreme Court ruled last month that Sharif had the right to return and the government should not try to stop him.
Nadir Chaudhri, another spokesman for the former prime minister, said he was planning to take part in elections due in the coming months.
“His plan is to go back to play his role in Pakistani politics, which is his right,” Chaudhri said.
“He’s head of his own party. Elections are coming up. He will mobilise his party for those elections.”
Public support for Sharif appears to be growing and he could become a potential obstacle to a power-sharing deal that Musharraf is discussing with Benazir Bhutto, also a former prime minister, that could see her return and the president quit as head of the army.