Pakistan has been led by President General Pervez Musharraf since Sharif's democratically elected government was toppled in a military coup in 1999. Following the coup, Sharif was arrested on corruption and terrorist charges, and then exiled to Saudi Arabia in 2000.
A spokesman for the Pakistan government claimed that before Sharif's release from prison in 2000, he had signed an agreement stating that he would take no part in Pakistani politics for the next 10 years.
Before his latest arrest at Islamabad airport, Sharif, who had stated his intention to begin a political fight against President Musharraf upon his return, had been planning to head for Lahore, where the headquarters of his opposition movement, the Pakistan Muslim league (PML) are based, said a PML representative.
The roads around Islamabad airport were closed off by police before Sharif's plane touched down, and there were reports of clashes between security forces and Sharif's supporters. There were also reports that more than 2,000 of the former prime minister's supporters had been arrested in the days leading up to his arrival.
Indirect presidential elections are planned for October in Pakistan, and general and provincial elections for January, 2008. Both Sharif and Benazir Bhutto, another exiled former prime minister, have stated their intentions of taking part in the 2008 elections.
Sharif, who led Pakistan between 1990-1993 and 1997-1999, was taken from Islamabad airport in a police bus, and a government spokesman stated that he had been arrested on corruption and money-laundering charges.
