A suicide car bombing outside the same consulate killed 12 Pakistanis and wounded 45 in June 2002. That attack was the work of Islamic fighters opposed to Pakistani cooperation in the US-led war on terror.
Police in the volatile port city said a van packed with explosives was towed away from the consulate to a sports ground where investigators defused it.
"If this exploded it would have caused massive destruction," Munir Ahmad Shaikh, a sub-inspector at the city's bomb squad told Reuters. "God has saved us."
A US embassy spokesman said consulate security personnel spotted the van and informed police before staff had arrived.
"Our people are not working, at least for today," he said.
A 750-litre drum containing a mixture of chemicals, including ammonium nitrate, was found in the van, police said.
Caught on camera
"If this exploded it would have caused massive destruction. God has saved us."
Munir Ahmad Shaikh,
Detonators were also found, but apparently they had not been connected to the drum of chemicals, police said.
Security cameras at the US mission had recorded a man parking the van, getting out of it and talking to a guard, police said.
"A youngster parked the van in front of the consulate, telling guards that it had broken down. He then drove away in another car," said Fayyaz Laghari, a deputy inspector general of police.
The van had been stolen in the city on Sunday evening by two gunmen who wounded the vehicle's owner when they grabbed it, police said.
Powell is due to visit the Pakistani capital, Islamabad but not Karachi, on Wednesday. Powell is visiting Pakistan on a tour that also takes him to India and Afghanistan.
In other developments:
Meanwhile, President Pervez Musharraf said on Monday a Libyan member of al-Qaida was behind two assassination attempts against him in December.
Recently there have been two
attempts on Musharraf's life (R)
"The man who organised the suicide attacks against me was from Libya and a member of al-Qaida," Musharraf told a meeting of tribal elders in the north-western city of Peshawar.
The Pakistani leader, who escaped the attacks unhurt, did not name the Libyan suspect, who he said funded Islamic fighters to carry out the bombings.
Musharraf has previously said al-Qaida could have played a part in the attacks, but that was the first time he explicitly identified a suspect.
"He gave 1.5-2 million rupees (US$26,100-US$34,700) to a Pakistani who recruited Islamic militants, Islamic extremists," Musharraf said.
The president promised the government would reveal more details about who was behind the attacks. He said the suspects would be shown on television.
"You will see their interviews," he said, without giving further details.
The two bombings happened 10 days apart in December. On both occasions Musharraf was travelling in a motorcade in Rawalpindi, near the capital Islamabad.
The first attack destroyed a bridge seconds after his vehicle had passed, but no one was hurt. In the second attack, suicide bombers tried to ram into his vehicle, killing 16 people.