"Nawaz Sharif is under pre-emptive arrest in connection with corruption charges against him," Sheikh Rashid, a railway minister and close confidant of general Pervez Musharraf, Pakistan's president, aid.
"I feel great, I'm prepared to face any situation," Sharif told Reuters as his flight landed in Islamabad.
Soldiers and police were on the tarmac awaiting the flight.
As officials boarded the aircraft and asked for his passport, the former prime minister insisted on personally presenting his papers at the immigration counter. He refused to be deported.
The supreme court ruled last month that Sharif had the right to return and the government should not try to stop him.
Sharif had been sentenced to life in prison on treason charges but released on condition that he live in exile for 10 years.
Geo TV, a private Pakistani television, said the government is likely to claim that Sharif's forced deportation has not violated any court ruling.
Unconfirmed reports said Sharif was headed for Jeddah and the Saudi authorities have agreed to host him for three more years.
"The high-handedness on show at the airport would go against the regime of General Pervez Musharraf," Sharif told Geo TV earlier.
He said his brother Shahbaz Sharif, who is in London, was to "to carry on the struggle in case he is detained". His brother was expected to return with him to Pakistan.
Police stopped Sharif supporters from
reaching the Islamabad airport [AFP]
A court in Lahore had issued an arrest warrant on Friday for Shahbaz in connection with a murder case.
Shahbaz condemned the former prime minister's arrest and subsequent deportation and said his "brother had been abducted by dictator Musharraf".
Talking to Al Jazeera, he said the treatment meted out to Sharif was "not only a blow to Sharif, but a blow to the dignity of the people of Pakistan".
Police arrested leading supporters of Sharif and clashed with others on blocked roads leading to to the airport.
Sohail Rahman, Al Jazeera's correspondent in Islamabad, said security forces had sealed off the airport and mobile phones have been jammed.
Sharif has made clear he plans to lead a campaign against Musharraf who toppled him from power eight years ago.
Sharif had insisted he would return on Monday despite calls for him to stay in exile from officials in Saudi Arabia and Saad Hariri, a Lebanese member of parliament.
"I have a duty, I have a responsibility, I have a national obligation to fulfil at all costs and that is democracy," Sharif said as he boarded his flight at London's Heathrow airport.
Sharif had planned 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 had prevented Sharif's supporters from entering or leaving the city on Sunday.
Ahsan Iqbal, a spokesman for Sharif, said on Sunday that authorities had detained more than 4,000 activists from his party in Punjab, the country's largest province where he commands a loyal following.
"Musharraf is shouting slogans of war against terror in order to win US support"
Ali Abu Hamza, Chichawatni, Pakistan
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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.
Muqrin bin Abdul Aziz, Saudi Arabia's intelligence chief, had 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.
Source: Al Jazeera and agencies