Nawaz Sharif was mobbed on his arrival at Islamabad
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After arriving at Islamabad airport he was charged with money laundering and put on a plane to Saudi Arabia.
Mr Sharif says he wants to challenge President Pervez Musharraf, who ousted him in a 1999 coup, in presidential elections due at the end of the year.
Mr Sharif had travelled to Pakistan weeks after the country's Supreme Court affirmed his right to return.
On board the plane which flew him to Islamabad from London, Mr Sharif told the BBC he wanted to help restore the rule of law.
"It's democracy versus dictatorship," he said.
Pakistani troops surrounded the plane after it landed.
There was a stand-off on board as Mr Sharif refused to hand over his passport to immigration officials for nearly two hours.
Eventually he agreed to leave the plane and was escorted to the airport's VIP lounge.
But shortly afterwards he was separated from his entourage, charged with money laundering and corruption, and flown to Jeddah in Saudi Arabia.
In Washington, the White House described Mr Sharif's deportation as an internal matter but added that the forthcoming election should be free and fair.
Saudi officials have indicated he will be allowed to stay in the kingdom.
Shots fired
Large numbers of police had set up barricades on roads to prevent Mr Sharif's supporters from reaching the airport.
There were reports of clashes between police and crowds of Mr Sharif's supporters in Islamabad, Rawalpindi, and Attok, where shots were fired and several people were said to have been injured.
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One worker from Mr Sharif's Muslim League party (PML-N) told the BBC he and about 20 others had been badly beaten by police outside the airport.
On Sunday, the party said more than 2,000 supporters had been arrested by the Pakistan authorities, while almost its entire leadership had been detained.
Supporters planned to launch a legal challenge to the deportation, which was "a violation of the court order under which Nawaz Sharif was allowed to arrive and stay in Pakistan," his aide, Sadique ul-Farooq, told the Associated Press.
The religious affairs minister said the government believed it was acting in accordance with the Supreme Court ruling.
Mr Sharif was exiled to Saudi Arabia in 2000 after being deposed, under what the government says was an agreement that he stay in exile for 10 years.
The former prime minister has denied there was ever such a deal.
Another former Pakistani Prime Minister, Benazir Bhutto, has said Mr Sharif's deportation does not affect her own plans to return soon to Pakistan.
Political crisis
The European Union Commission said Mr Sharif should be allowed to defend himself against court charges in Pakistan.
Mr Sharif's supporters were kept away from the airport
"In our view the Supreme Court's ruling is very clear and should be respected," said a spokeswoman.
Mr Sharif had planned to lead a motorcade to Lahore, his political power base, but he was aware he might not be allowed the opportunity.
He decided at the last moment to leave his brother Shahbaz behind in the UK "to hold the fort" in case he were jailed or deported.
For the army, a decision to arrest Mr Sharif is as much a political as a legal decision, says the BBC's M Ilyas Khan.
The military does not want to make Mr Sharif into a political martyr but it also does not want to see him campaigning for power, he says.
Gen Musharraf has been struggling to contain protests that have grown since he tried to remove Supreme Court Chief Justice Iftikhar Muhammad Chaudhry.
The president plans to seek a new five-year term in office in the forthcoming election.
Carte du Pakistan

L'ancien premier ministre pakistanais, Nawaz Sharif, a été incarcéré dès son arrivée au Pakistan. Après avoir été renversé en octobre 1999 à la suite d'un coup d'État perpétré par le général Pervez Musharraf, actuel président du pays, il était parti en exil en Arabie saoudite. Quelques heures après son arrestation, Sharif était expulsé vers Djeddah en Arabie saoudite.

Les autorités du pays lui reprochent détournements de fonds, fraude fiscale et trahison. Condamné à la prison à vie, un accord est survenu avec les putschistes pour qu'il puisse partir en exil pendant 10 ans. C'est à la suite de la déclaration d'illégalité de cet accord par la Cour suprême, qu'il a été autorisé à rentrer dans son pays.

L'ancien premier ministre voulait participer aux futures élections législatives fin 2007-début 2008, s'opposant ainsi au pouvoir en place.