Hashmi, the president of the Alliance for the Restoration of Democracy, was taken into custody late Wednesday night.
The BBC said Hashmi is one of the most outspoken critics of Pakistan President Pervez Musharraf, an army general who seized power in a 1999 coup.
Hashmi is also the acting leader of a faction of the Pakistan Muslim League associated with Nawaz Sharif, the former prime minister who was overthrown during the military coup.
The government issued a statement saying Hashmi was arrested because he had "maligned Pakistan army personnel and attempted to incite them to mutiny."
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