Pakistan's Interior Ministry on Tuesday placed cleric Hafiz Saeed under "preventative detention." Saeed is head of Jamaat-ud-Dawa (JuD), an Islamic charity the UN lists as a terrorist organisation.
Police detained Saeed Monday evening. On Tuesday morning he was escorted to his residence in Lahore where he was put under house arrest.
"My detention orders are unlawful and we will challenge them in the court," Saeed told reporters before he was led away by police.
Supporters of Saeed plan protests against his detention in major Pakistani cities on Tuesday.
Saeed founded Lashkar-e-Taiba (LeT), a militant group blamed for the 2008 attack on India's financial capital, Mumbai in which 10 gunmen killed 166 people in an assault on two luxury hotels, a Jewish centre, and a train station.
JuD is banned in Pakistan and considered a front for LeT by the US and India.
New Delhi has been demanding action against Saeed since the Mumbai attack. Why Pakistan acted now is unclear.
A senior Pakistani defence ministry official said Islamabad had been feeling US pressure on the issue, although the administration of President Donald Trump had not been in contact.
"Trump is taking hard decisions against Muslim countries, there is open talk of actions against Pakistan also. So yes, this was a consideration," said the official.
TRT World spoke to Kamran Yousaf in Islamabad and Tavleen Singh in New Delhi for more details.