More than 300 party workers were arrested in Lahore, and about 1,000 across the country, said a statement by the Pakistan Muslim League (N).
A deal negotiated between the Pakistani and Saudi governments arranged for the Sharif brothers to live in Saudi Arabia. But last year Shahbaz came to America for treatment and refused to return to Saudi Arabia. He is now in Britain and has pledged to return to Pakistan to "live and serve the people."
Political analysts ts say if allowed to return, Shahbaz, who has many followers in the Punjab, can cause problems for the Musharraf government, which is urging him not to return.
Shahbaz plans to return this week.
His move has panicked the government, which has ordered a countrywide crackdown on party workers to prevent them from coming to receive Shahbaz in large numbers.