Mr Hashmi has been critical of the country's military ruler
Opposition parties in Pakistan have condemned the arrest of one of their leaders, Makhdoom Javed Hashmi.
The decision to detain Mr Hashmi was almost universally attacked as an affront to democracy.
Mr Hashmi, the president of the Alliance for the Restoration of Democracy (ARD), was arrested late on Wednesday night and accused of defaming the army.
He had earlier claimed to have received a letter from the country's armed forces purportedly criticising President Musharraf's role in the US-led "war on terror".
Mr Hashmi is one of the most outspoken critics of President Musharraf, an army general who seized power in a coup in 1999.
As well as heading the ARD, Mr Hashmi is acting leader of a faction of the Pakistan Muslim League associated with Nawaz Sharif, the former prime minister overthrown in the military coup.
Mr Hashmi's daughter, Memona, said the arrest was unlawful.
was dragged, beaten and thrown in a vehicle like a
criminal," she told the Associated Press.
A large group of MPs from different opposition groups marched on the parliament building in the capital, Islamabad, on Wednesday to protest against Mr Hashmi's arrest.
Angry speeches accused the government of victimising him.
The opposition is dominated by hardline Islamists and factions allied to Mr Sharif and to another former prime minister, Benazir Bhutto.
Information Minister Sheikh Rashid Ahmed told Reuters that Mr Hashmi's arrest followed complaints that he had forged a letter from the Pakistani armed forces.
An official statement said Mr Hashmi was under arrest because he had "maligned Pakistan army personnel and attempted to incite them to mutiny".
Pakistani newspapers reported last week that Mr Hashmi claimed to have a letter from military officers praising the opposition for blocking a US request for Pakistani troops to be sent to Iraq.
The letter, apparently unsigned and written on military stationery, is also said to have criticised President Musharraf's decision to back the American campaign in Afghanistan.
President Musharraf's support in the military has been questioned
It allegedly called for an inquiry into the 1999 coup, and into the Kargil conflict with India that preceded it.
Government authorities say Mr Hashmi faked the letter to undermine Pakistan's stance against its nuclear rival and neighbour, India.
Pakistan's Government, which supports President Musharraf, frequently collides with opposition parties who accuse the country's military leader of suppressing democracy.