The emergence of the ''Defense of Pakistan Council'' movement has raised suspicions that the group has approval from elements in the powerful military and security establishment, aiming to bolster public support for a hardline position.
The group's rise comes as the military is trying to assert its position in renegotiating its troubled relationship with the United States and as Pakistan prepares for elections likely to take place later this year.
Some of the leading lights in the Defense of Pakistan Council have traditionally been seen as close to the security establishment, which has a long history of propping up radicals to defend its domestic interests or fight in India and Afghanistan.
On Sunday, the group's bandwagon rolled into Karachi, the country's commercial heart.
Between 20,000 and 30,000 men gathered close to a monument to Pakistan's founder, Mohammed Ali Jinnah, whose vision of a liberal, secular Pakistan is often contrasted to the rise of hardline, often violent groups in the country.
The star of the gathering was Hafiz Saeed, the head of Jamaat-ud-Dawa, a front group for Lashkar-e-Taiba, the militant group accused by India and the West of sending militants by boat to Mumbai in 2008 where they killed 166 people in attacks on a hotel and other sites.
''We demand Pakistani rulers quit the alliance with America,'' said Saeed, who was placed under house arrest after the Mumbai attacks but was released later and has slowly re-emerged in public now, without a response from authorities. ''There can be no compromise on the freedom and sovereignty of the country.''
Members of Dawa patrolled the rally, some armed with automatic weapons, others on horseback.
Also represented on stage and in the crowd were Sipah-e-Sahaba, a feared Sunni extremist group that has carried out scores of attacks on minority Shias in recent years. Its members have reportedly formed alliances with al Qaeda operatives in Pakistan.
A large banner that hung over the stage read ''Wake up, countrymen, break the shackles of American slavery.''
That anti-American message has been amplified by the Pakistani army since US airstrikes along the Afghan border in late November killed 24 Pakistani soldiers.
The Pakistani army accused the US of deliberately targeting the outposts, rejecting American assertions it was mistake.
Pakistan retaliated by closing its western border to Nato and US military supplies into Afghanistan, a key supply line for the war.
Saeed and other speakers threatened civil disobedience if Pakistan reopens it. Their stance could hamper American hopes that Islamabad will quietly reopen the route in the coming weeks.
''We vow that the Nato supply will never be restored,'' he said.
The alliance groups many of the same parties and clerics that banded together after the Sept. 11, 2001 attacks, capitalising on anti-American sentiment.
The current government, which doesn't espouse political Islam, is under pressure from the courts and opposition parties.
Elections are now seen as likely later this year, and the revival of the ''Defend Pakistan'' group appears to be a push by politicians grouped within it to win votes among the legions of Pakistanis who subscribe to Islamist views. However Hafiz Saeed has reportedly denied that his party will take part in election.
It could also be attempt by the army to put pressure on the ruling Pakistan People's Party, which has repeatedly clashed with the generals since taking power in 2008 and has tried to get closer ties with India.
The group has organised large rallies in several cities; next week it plans a gathering in the capital, Islamabad.
Many of the speakers in Karachi rallied the crowds with warnings that Pakistan was under threat, and Islam its only defense.
''Do you swear to fight back with Islamic spirit, honour and dignity if anyone, whether American, Nato, Israel or India attack Pakistan?'' asked Maulana Sami-ul-Haq, head of a hardline school that has sent thousands of people to fight in Afghanistan over the last 10 years.
''Jihad! Jihad!'' the crowd roared.
Speaker after speaker also touted the army line on India, saying the neighbouring country represents an existential threat to Pakistan.
This stance justifies the security state that has been established since the two nations were partitotioned under a British-ruled subcontinent in 1947.
The two nuclear-armed neighbours had fought three wars since their independence in 1947, 1965 and 1971.
Liberals, democrats and peace activists have been trying for years to bring India and Pakistan closer together.
But in the past, the army has funded and trained militant groups and their umbrella organisations to battle Indian forces in Kashmir, the disputed territory at the heart of the rivalry between the two countries after partition.
"The security establishment of this country desires that ultra-radical parties should be brought into politics so that their doctrine against India, America or Israel could be infused to the masses," said Tauseef Ahmed, the head of the Mass Communication department at the Federal Urdu University.
Also at the Karachi rally was Hamid Gul, a former general who headed the country's spy agency in the late 1980s when Pakistan and the US were supporting militants in their fight against the Soviets in Afghanistan.
He has since become a leading voice in the media against America and in support of the Taliban. Documents released by the whistleblower site Wikileaks alleged he retained ties to the insurgency there, a charge he denies.
Ejaz Haider, a security analyst, said the security establishment should be "checked for serious dementia" if it was using the council for its own purposes, given that many of its members have been linked to terrorism that is taking a deadly toll inside Pakistan.