Hundreds of thousands of people have been greeting Benazir Bhutto, following her dramatic return to Pakistan after eight years in exile.
Well-wishers packed the streets of Karachi in the hope of glimpsing the ex-PM, causing her motorcade to get bogged down as it left the airport.
Ms Bhutto is set to hold power-sharing talks with President Pervez Musharraf, which could see her becoming PM again.
She has pledged to work towards free and fair elections.
There is a huge security presence amid threats of attacks by militants.
Ms Bhutto flew from Dubai, accompanied on the flight from Dubai by about 100 members of her Pakistan People's Party (PPP).
At least 200,000 people turned out to greet her, lining the 6km (four-mile) route from the airport into the city.
The BBC's Syed Shoaib Hasan in Karachi says many people are dressed in the red, black and green colours of Ms Bhutto's PPP party and there is a carnival atmosphere, with music playing and food stalls in place.
The size of the crowd on the streets means her convoy is making very slow progress.
Reports say her special bullet-proof truck took six hours to go just 5km.
The former prime minister rejected police calls to sit inside the vehicle, instead on top of it with party colleagues.
Ms Bhutto is planning to make a speech at the tomb of Pakistan's founding father, Mohammed Ali Jinnah.
"It's an extremely moving and emotional moment for me," Ms Bhutto told the BBC after her arrival in Karachi, Pakistan's largest city.
"I could not believe that this day that I have counted the hours, the minutes, the months and the years to, had finally arrived."
When she first stepped onto Pakistani soil at Karachi airport Ms Bhutto was overcome with emotion and burst into tears.
06 Oct: Presidential polls held
17 Oct: Supreme Court resumes hearing challenges to Musharraf candidacy
18 Oct: Benazir Bhutto's homecoming
15 Nov: Parliamentary term ends and general election must be held by mid-January
Many PPP supporters have been bussed in from outlying areas by the party - a show of organisational strength which the ex-prime minister hopes will boost her chances of a return to power, correspondents report.
The BBC's Damian Grammaticas in Karachi says that despite being away for eight years, Ms Bhutto is still enormously popular as a scion of Pakistan's pre-eminent political dynasty.
About 20,000 troops and police have been deployed in the city to oversee Ms Bhutto's arrival amid threats by Islamist militants to assassinate both her and Gen Musharraf.
Gen Musharraf had asked Ms Bhutto to delay her return until the Supreme Court decided whether he was eligible to serve as president for another term.
He easily won a presidential vote on 6 October after opposition deputies in the national and provincial assemblies - which choose the president - either boycotted or abstained from the vote.
However, the court said he could not be officially declared the winner until it had finished ruling on objections to his candidacy.
Ms Bhutto left the country soon after Gen Musharraf seized power in a coup.
Washington has backed a power-sharing deal with Gen Musharraf which would see Ms Bhutto becoming prime minister.
It has become increasingly concerned over the military's inability to defeat Islamist extremists and Gen Musharraf's rising unpopularity.