Pakistan stepped up airport security, banned rallies and continued rounding up hundreds of opposition activists Sunday, a day before former Prime Minister Nawaz Sharif's planned return to challenge the military ruler who sent him into exile seven years ago.

A researcher for Human Rights Watch said the group was "deeply concerned about the deteriorating situation for civil and political rights in Pakistan."

"We are aware of the fact that hundreds of activists of the Pakistan Muslim League Nawaz group have been arrested in the run up to Nawaz Sharif's arrival in Pakistan," Ali Dayan Hasan, South Asia researcher for Human Rights Watch told AP Television.

"These arrests are still underway. People have been arrested under the maintenance of Public Order act under the anti-terrorism act and sometimes just detained without charge," he said.

More than 2,000 Sharif supporters in Punjab have been detained in a crackdown over the past four days, and others have gone into hiding, said a spokesman for Sharif's party.

Police and security officials, speaking on condition of anonymity because of the sensitivity of the issue, confirmed about 700 arrests.

"We would like these people to be released immediately and we would like the government to understand that this sort of curb on the freedom of expression and the freedom of association is no longer acceptable in Pakistani courts or on the Pakistani streets," Hasan said.

Sharif is scheduled to fly to Islamabad on Monday to campaign against President General. Pervez Musharraf who ousted Sharif's elected government in a 1999 coup setting up a political showdown that could spark further unrest.

In a television interview broadcast Sunday, Sharif acknowledged he might be arrested on his return on corruption charges from his days in power in the 90s.

Media reports said he may be deported on arrival, but the government has not said what action, if any, it plans to take against Sharif.

The looming showdown, which could further weaken Musharraf's faltering grip on power, comes as the country battles surging
Islamic extremism that has spread from the Afghan border, where Osama bin Laden and other al-Qaida leaders are believed to be hiding.

Supporters fear Sharif could be arrested or deported on his arrival. The government has dusted off corruption cases against him, and media reports suggest a "VIP cell" at a 16th-century fortress is being readied.

A court issued an arrest warrant last week in connection with a murder case for his brother Shabhaz Sharif, who will return with him. Shabhaz Sharif has denied the allegation.

Analysts say Sharif's return could upset talks on a power-sharing pact between his archrivals Musharraf and Benazir Bhutto, another exiled former premier plotting a political comeback in the country, which is a key U.S. ally in the war on terror.

Musharraf wants to win a new five-year presidential term from lawmakers by mid-October. Both Sharif and Bhutto want to contest general elections due by mid-January 2008.

But the general has seen his popularity shrink since his failed attempt to fire the country's top judge earlier this year spurred calls for an end to military rule.

His administration is also struggling to contain a surge in Islamic militancy that has spread from the Afghan border, where Osama bin Laden and other al-Qaida chiefs are believed to be hiding.

Sharif plans to return with his brother Shabhaz Sharif, who last week had an arrest warrant issued against him in connection with a murder case.

After arriving in Islamabad, the Sharifs plan to travel in a motorcade to their home and political base in Lahore, about 290 kilometres (180 miles) to the south.

The trip through Punjab province could take three days as he greets supporters along the way, Sharif's party said.

Security has been stepped up at all of Pakistan's major airports after receiving reports about possible terror attacks, two senior intelligence officials said on condition of anonymity because of the sensitive nature of the issue.