Hafiz Mohammed Saeed -- the leader of Pakistan's Jamaat-Ud-Dawa (JUD) -- was detained in Punjab, Pakistan's largest province, according to a government statement released late Sunday.
Saeed is under detention in the town of Lahore. Four other men -- Abdullah Ubaid, Zafar Iqbal, Abdur Rehman Abid and Qazi Kashif Niaz -- have also been placed under house arrest in other towns around Punjab.
The arrests come in conjunction with the placing of two organizations -- JUD and the Falah-e-Insaniat Foundation -- on a watch list under Pakistan's Anti-Terrorism Act of 1997.
All the men were detained for being active members of the organizations and are being held under Section 11-EEE of the Anti-Terrorism Act, which gives the government the power to arrest or detain suspects for up to 12 months.
Speaking after the arrests, a spokesman for India's Ministry of External Affairs greeted the news with caution.
"Exercises such as yesterday's orders against Hafiz Saeed and others have been carried out by Pakistan in the past also," he said in a statement.
"Only a credible crackdown on the mastermind of the Mumbai terrorist attack and terrorist organizations involved in cross border terrorism would be proof of Pakistan's sincerity."
US blamed for pressuring Pakistan to make arrests
In a video statement released by the JUD, Saeed said the decision to arrest him had been made due to "foreign pressure."
"(Donald) Trump is the new president of the United States who wants to have good relations with (Indian Prime Minister Narendra) Modi," said Saeed.
JUD spokesman Hafiz Khalid Waleed also blamed the US for "putting pressure" on Pakistan to get Saeed arrested, threatening court action as well as street rallies. "We are going to take this matter to the courts and to the streets. We are planning further rallies across the country in protest of this action," Waleed said, adding that Saeed's house arrest is for 19 days.
Neither Washington nor India have commented on the accusations, but security analyst and director of the Pak Institute for Peace Studies, Amir Rana, says it's more likely the decision was an internal one.
"This arrest was a long time coming," he says, adding that Saeed has been put under house arrest on two previous occasions, once in 2001 and again in 2003.
"This appears to be a genuine effort by Pakistan authorities to combat non-state actors within its borders," Rana told CNN.
India has long called for Saeed's arrest, for what it says is his involvement in a number of terror attacks on Indian soil. In the past, India says it has provided irrefutable proof of Saeed's involvement in the Mumbai attacks.
Saeed is accused by both India and the US of being responsible for the deadly attacks
that struck different locations across Mumbai nearly a decade ago, when a group of gunmen went on a three day rampage, using automatic weapons and grenades to strike hotels, cafes, a railway station and a cinema.
The outlawed group, Lashkar-e-Tayyiba -- which means "army of the pure" -- was widely thought to be responsible
for the attacks. According to the UN Security Council, Saeed's JUD organization is synonymous with the terrorist group and supportive of al Qaeda and the Taliban.
Saeed denies involvement in Mumbai massacre
Saeed has repeatedly denied any involvement in the Mumbai killings or support for terrorist activities. "Those people who are carrying out violent attacks or are involved in militants activities ... are giving the mujahadeen and jihad a bad name," he said in an interview with CNN in 2012
"I condemn this attack and the innocent lives that have been lost."
He also highlighted Jamaat-Ud-Dawa's charitable activities and extended an offer of aid to the US in the wake of Superstorm Sandy in 2012 -- an offer that was quickly rejected by Washington.
The US State Department describes JUD's mission as the establishment of Islamist rule in India and Pakistan and is offering as much as $10 million
for any information that might lead to his arrest or conviction.