Authorities have rounded up hundreds of supporters of two-time prime minister Nawaz Sharif and his politician brother, Shahbaz, apparently in the hope of thwarting big welcome rallies if and when the two end seven years of exile on Monday.

The return of the prime minister he ousted in 1999 is a serious challenge for Musharraf, whose popularity has been sliding as he prepares to try to get re-elected by the national and provincial assemblies between September 15 and October 15.

He aims to hold a general election around the year-end.

"There's complete panic in government ranks," said a spokesman for Sharif, Ahsan Iqbal.

With political tension rising, a senior U.S. official arrived to prepare for a scheduled "strategic dialogue" session next week. U.S. Assistant Secretary of State Richard Boucher is likely to meet Musharraf for talks.

The United States is keen to ensure nuclear-armed Pakistan sustain efforts in the global war on terrorism and has been encouraging army chief Musharraf to work with liberal-minded former prime minister Benazir Bhutto.

Musharraf has been negotiating with Bhutto on a package of proposals that would likely see Musharraf quitting his post as army chief to become a civilian president, and Bhutto being cleared of corruption charges and coming back for elections.