A court ordered former Pakistani president Pervez Musharraf to be detained for 14 days on Friday, the latest twist in a long-running feud between the one-time army chief and the judiciary.
The order followed his arrest on Thursday over accusations that he was responsible for a 2007 raid on a radical mosque that killed more than 100 people, police said.
The arrest came a day after his lawyers said he had been granted bail in other three cases and was cleared to leave the country.
The complaint against Musharraf in the Red Mosque case was registered last month on the orders of a judge. Pakistani troops stormed the mosque after a stand-off with hardline Islamists inside.
Musharraf, who has denied wrongdoing in the Red Mosque deaths, took power in a 1999 coup but was ousted by popular protests led by the country's judiciary in 2008.
He went into self-imposed exile the same year to live in Dubai and London but returned earlier this year in an abortive attempt to launch a political career before general elections in May.
His arrest comes as an increasingly activist judiciary has challenged the power of the powerful military. Apart from Musharraf, none of Pakistan's army chiefs has been arrested, even after leaving office.