Radical cleric Hafiz Saeed was released on Friday after less than a year under house arrest.
A decision to release Saeed came from the Lahore High Court, which rejected a government plea seeking to extend his detention for another three months.
The court said the government had failed to provide sufficient evidence to justify the extension on the grounds that Saeed's release would jeopardize public safety.
"Hafiz Saeed is a Pakistani citizen and the court has released him. He didn't commit any crime and there was no legal justification for his house arrest," Ahmed Nadeem, Saeed's Jamaat-ud-Dawa (JuD) group's deputy information secretary, told DW.
"Pakistan is a sovereign country and should not tolerate pressure from any other country over Saeed," Nadeem added.
In 2008, the world watched on as militants launched a gun and grenade assault on India's financial capital, which lasted a full three days.
The court decision on Wednesday caused uproar in India, which has long accused Pakistan of failing to prosecute or hand over those accused of plotting the attacks.
India has also claimed to have proof that "official agencies" in Pakistan were involved in the planning, charges Islamabad vehemently denies.
India's ministry of external affairs (MEA) said Hafiz Saeed's release confirmed once again the lack of seriousness by the Pakistan government in bringing to justice perpetrators of terrorism, including individuals and entities designated by the UN.
"It also appears to be an attempt by the Pakistani system to mainstream proscribed terrorists. Pakistan has not changed its policy of shielding and supporting non-state actors and its true face is visible for all to see," MEA spokesperson Raveesh Kumar told a press conference.
"Why are we surprised [at Saeed's release]? The judicial process in Pakistan is a joke. The prosecuting and investigative agencies must have brought no credible evidence [against Saeed]," Ajay Sahni, an Indian counter-terrorism expert, told DW.
G. Parthasarathy, the former high commissioner to Pakistan, said that after former Pakistani PM Nawaz Sharif's ouster, "the military generals are calling the shots in Pakistan."
Saeed has long campaigned in support of Muslim separatists in the Indian state of Jammu and Kashmir, to which Pakistan also lays claim.
He heads the JuD Islamist charity, which is alleged to be a front for the Lashkar-e-Taiba (LeT) militant group. LeT is among several jihadi groups fighting Indian troops in the disputed Kashmir region, while allegations continue to spread that Pakistan's security establishment is aiding militant groups in the area.
In a video message after the court decision, Saeed claimed that he was detained because of his associations with the disputed Kashmir region. "It's because of Kashmir that India is after me, but all her efforts have been in vain and Allah has set me free," he said. "This is a victory of Pakistan's freedom and God willing Kashmir will also be freed because I'm fighting Kashmir's case."
Ayub Malik, an Islamabad-based security analyst, says the JuD will now speed up its political activities, which will further damage Pakistan's ties with India.
"At the same time, Saeed could be dangerous for Pakistani society because he will spread religious hatred openly. It is not good for a country that is experiencing an extremist surge," Malik told DW.
US bound to exert pressure
Saeed's release was expected to see Washington mount increasing pressure on Pakistan for its alleged support for militant groups, including the Taliban in Afghanistan.
In August, US President Donald Trump accused Pakistan of harboring "agents of chaos," while Secretary of State Rex Tillerson just last month said that the White House had made "very specific" requests to Pakistan on combating militancy in the region.
The US has also made several overtures under Trump for closer relations with Pakistan's regional rival, India.
The US is expected to keep its $10 million (€8.5 million) bounty on Saeed's head for his alleged involvement in the Mumbai attack, in which six US citizens were also killed.