Pakistani stocks fell 4.2 per cent on Wednesday amid fears the country is heading towards another period of political instability.
Shahbaz Sharif was banned from standing for election over allegations he was involved in the extra-judicial killing of five anti-government fighters in 1998.
A court later acquitted him of the charges, clearing the way for him to stand in the 2008 elections.
Nawaz Sharif was barred from participating in last year's election because he had been convicted for the 1999 hijacking of the then army chief General Pervez Musharraf's aircraft.
The incident triggered a military coup led by Musharraf and Sharif, who served two terms as prime minister during the 1990s, was subsequently ousted from power.
He returned from exile last year to lead his Pakistan Muslim League (Nawaz) during the 2008 general election.
The election ban prevents Sharif from assuming a leadership position in parliament.