"My group claims responsibility for the Karachi attack and we will carry out more such attacks, within 10 days,” Asmatullah Shaheen, one of the commanders of Tehrik-i-Taliban Pakistan (TTP), or Taliban Movement of Pakistan, who spoke by telephone to a Reuters reporter in Peshawar.
The prospect of more violence comes at a tough time for embattled President Asif Ali Zardari. He already faces political pressure because corruption charges against some of his aides may be revived.
And Zardari has yet to formulate a more effective strategy against the Pakistani Taliban, despite relentless pressure from Washington, which wants his government to root out militants who cross over to attack US and Nato-led forces in Afghanistan and then return to their Pakistan strongholds.
The scale of his challenges was clear on Monday, when a suicide bomber defied heavy security around a Shia procession, killing 43 people and triggering riots.
In a sign of mounting frustrations, Pakistani religious and political leaders called for a strike for Friday to condemn that attack, one of the worst in Karachi since 2007.
The bloodshed illustrated how the Taliban, whose strongholds are in the lawless northwest, have extended their reach to major cities in their drive to topple the government.
“The bombing itself was bad enough, but the violence that immediately erupted was also very well planned,” said Sunni scholar Mufti Muneebur Rehman, who blamed Pakistani authorities for the chaos.
“We want the government not only to compensate those killed in the attacks, but also those who lost their livelihoods, and so we are calling for a complete strike on Friday,” he said.
The Taliban campaign and their hardline brand of Islam — which involves public hangings and whippings of anyone who disobeys them — angered many Pakistanis.
But the Karachi bomb suggested growing violence has raised suspicions of Pakistan's government.
“The government is using the Taliban as an excuse for everything that is happening anywhere in the country,” said Noman Ahmed, who works for a Karachi clearing agency.
“The organised way that all this is being done clearly shows that the terrorists are being sponsored either by the government itself or some other state that wants to destabilise Pakistan.”
Pakistan's all-powerful military sets security policy. So the key gauge of public confidence may be how the army's performance is viewed. In the 1980s, Pakistan's army nurtured militant groups who fought Soviet occupation troops in Afghanistan. The Taliban emerged in the 1990's after a civil war in Afghanistan.
Now Pakistan's army faces home-grown militants.
“I don't buy that foreign hands are involved (in the Karachi attack). They're domestic elements. They're those who were nurtured, trained and protected in late 1990s,” said Sajid Ali Naqvi, head of the influential Shias' Islami Tehrik movement.
The bombing was one of the bloodiest in Karachi since an October 2007 attack on former Prime Minister Benazir Bhutto on her return to the country that killed at least 139 people.
Shia leaders, as well as Karachi's dominant Muttahida Qaumi Movement (MQM) political party, backed the strike call, which could bring the teeming city of 18 million to a standstill.
The high-profile bloodshed had all the hallmarks of the Taliban, who often bomb crowded areas to inflict maximum casualties. The blast led some Pakistanis to conclude that several hands must have been involved.
“The Taliban, or whoever is behind this, cannot do it without the support of a government,” said Shahid Mahmood, whose perfume and watch shops were torched in the riots.
“They know that Karachi is the heart of Pakistan and if it goes down, the country will go down.”