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.

Sharif plans to fly to Islamabad on Monday to campaign against President Gen. Pervez Musharraf — who ousted Sharif's elected government in a 1999 coup — setting up a political showdown that could further shake Musharraf's weakening grip on power and spark unrest.

"I will go back to Pakistan on Sept. 10 with my brother because my country needs me," Sharif said Saturday at a news conference in London. "I am going to lead the people of Pakistan against the dictatorship, and the dictator sitting in Islamabad should give up his futile efforts to stop me."

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. The government has not said what action, if any, it plans to take against Sharif.

"I know that this is a risky course for me and there can be dangers in it for me. But I am doing this for Pakistan," he told Pakistan's Geo TV channel.

"Nothing else can be more pleasing for me then freeing (Pakistan) from the clutches of military dictatorship. I will be happy that for a small price — my going to jail — Pakistan will get freedom," he said.

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.

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.

After arriving in Islamabad, the Sharifs plan to travel in a motorcade to their home and political base in Lahore, about 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.

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, party spokesman Ahsan Iqbal said. Police and security officials, speaking on condition of anonymity because of the sensitivity of the issue, confirmed about 700 arrests.

Human Rights Watch urged Musharraf to release the detainees.

"The government should release arbitrarily detained opposition activists immediately and allow them to peacefully welcome Nawaz Sharif freely and without threat of violence," said Ali Dayan Hasan, South Asia researcher for the rights group.

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.

The new security measures came days after the U.S. Embassy warned its citizens to avoid popular markets and crowded areas, saying it had received "non-specific information regarding terrorist attacks, possibly suicide attacks, against U.S. interests or places frequented by Westerners in the major cities in Pakistan."

The government will take action against anyone violating a ban imposed on rallies in Rawalpindi, a city near Islamabad where the federal capital's airport is located, Rawalpindi Mayor Raja Javed Ikhlas said.

He said the ban came in response to two homicide bombings that killed 25 people and wounded more than 60 others Wednesday in the garrison city.

In an unexpected development on Saturday, two foreign envoys — Lebanese lawmaker Saad Hariri and Saudi intelligence chief Prince Muqrin bin Abdulaziz Al Saud — appeared at a news conference outside Musharraf's presidential office saying Sharif should respect a 2000 agreement under which he allegedly promised to stay away from Pakistan for 10 years.

"We are sincerely hoping that his excellency Nawaz Sharif honors that agreement," the Saudi envoy said, adding King Abdullah was concerned about the "unity, stability and prosperity" of Pakistan.

Sharif acknowledged that Hariri — the son of assassinated former Lebanese Prime Minister Rafik Hariri — helped broker his release in 2000 after he was convicted of terrorism and hijacking charges in Pakistan, and the agreement included a 10-year exclusion period.

But Sharif claimed that Hariri later told him the period of exile was only five years, though he conceded that was not mentioned in the document he signed.

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.

Musharraf 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.

Musharraf has been holding talks with Bhutto to reach a deal that could see the bitter rivals share power. But the negotiations appear to have run into opposition from lawmakers on both sides.

Bhutto says she also plans to return to Pakistan, regardless of the outcome of the talks. She is due to announce her return date on Sept. 14.