Former Pakistani Prime Minister Nawaz Sharif has disembarked from a plane at Islamabad airport after a delay following his arrival from London.
Police have sealed off roads leading to Islamabad airport
Mr Sharif, who was ousted by President Pervez Musharraf in a bloodless coup in 1999, has spent seven years in exile.
Troops surrounded the plane, and there was a stand-off on board as Mr Sharif refused to hand over his passport.
Large numbers of police have set up barricades on roads to prevent his supporters reaching the airport.
As his plane landed, Mr Sharif told Reuters news agency: "I feel great, I'm prepared to face any situation."
On Sunday, Mr Sharif's Muslim League party (PML-N) said more than 2,000 supporters had been arrested by the Pakistan authorities, and several hundred more were picked up in the Rawalpindi-Islamabad region overnight.
A provincial police official admitted to detaining several hundred "trouble-makers".
Police have fired teargas shells to disperse crowds of Mr Sharif's supporters in Rawalpindi, the BBC's M Ilyas Khan reports.
Almost the entire leadership of the party there has been detained by the police, and dozens more were arrested while attempting to lead party workers towards the airport on Monday morning.
Mr Sharif says he is determined to lead a campaign against General Musharraf ahead of elections.
The Supreme Court ruled last month that Mr Sharif had the right to return to the country, but the government has urged him to honour the terms of an exile deal which, it says, bars him from coming back for another three years.
Mr Sharif has denied agreeing to such a deal.
A group of political supporters and journalists travelled with him on the Pakistan International Airlines flight, whose departure from London was delayed by more than one hour as a passenger was apparently taken ill.
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
10 Sep: Nawaz Sharif boards flight home
Mr Sharif's aides changed the flight at the last moment in an apparent effort to outwit the Pakistani authorities.
The former prime minister wants to lead a triumphal motorcade from Islamabad to Lahore, his political power base, but the government has hinted that he may be arrested - or even deported.
Before setting off on Sunday, Mr Sharif said he was not scared of the possibility that General Musharraf would send him straight to jail.
"He kept me in prison for 14 months after staging a coup d'etat against my government and I'm struggling from that very day," the former prime minister said.
"I have a duty, I have a responsibility, I have a national obligation to fulfil at all costs and that is democracy," he told the BBC's Owen Bennett-Jones who is travelling with him.
However, he decided at the last moment to leave his brother Shahbaz, also a politician, behind "to hold the fort" if General Musharraf jails the former prime minister.
Mr Musharraf has made no secret of his contempt for Nawaz Sharif, describing him as corrupt and incompetent.
Pakistan authorities hope to prevent large-scale rallies
But for the army, a decision to arrest him is as much a political as a legal decision, says our correspondent.
The military do not want to make Nawaz Sharif into a political martyr but they also do not want to see him campaigning for power, he says.
The government has said his return would destabilise the political environment ahead of a general election which is due in coming months.