The Taliban, determined to topple the government, claimed responsibility for Monday's attack on a huge crowd of Shi'ite Muslims and threatened more bloodshed.
The prospect of increased violence comes at a delicate time for President Asif Ali Zardari, who faces political heat because corruption charges against some of his aides may be revived.
Karachi's streets were nearly empty. The stock exchange, which normally operates on the first day of the year, was closed.
Police have arrested 18 people since riots triggered by the bombing destroyed hundreds of shops, costing Pakistan's biggest city an estimated 30 billion rupees ($356 million) in damages.
"We have no information of any specific threats for today," Karachi police chief Waseem Ahmed told Reuters. Nevertheless, police and paramilitary forces carried out patrols.
Residents were not taking any chances, fearful of new attacks by militants who have killed hundreds of people since October, despite a security offensive in one of their main strongholds.
"We are already losing business and can't take the risk of going out today and opening our shops," said Saleem Ahmed, who sells electronics at one of the city's main markets.
"If something happens or anyone comes and damages, say, one refrigerator or deep freezer, I will lose more money than what I would have earned the whole day, so I better stay home," he said.
In a sign of growing anxiety over Taliban and al Qaeda attacks, the United Nations plans to move some of its international staff out of Pakistan, a spokeswoman said on Thursday.
The bloodshed is also frustrating many Pakistanis.
Hundreds of shops were torched in the rampage that followed the bombing that hit Karachi, a teeming city of 18 million.
According to officials, Karachi generates 68 percent of government revenue and 25 percent of Pakistan's gross domestic product. It is also home to two of the country's main ports.
While investors in Pakistan have got used to almost daily violence in the northwest, bloodshed in Karachi has a much more direct impact on financial markets and investor sentiment.
Analysts fear further attacks here could raise doubts about the prospects of recovery for an economy in virtual recession.