Employees of some of the most famous financial institutions in the US have been urged to report for work despite "credible" threats from al-Qaeda.
New York's Stock Exchange is said to be on the list of targets
The security alert has been raised to "high" - the second-highest level - in parts of Washington DC and New York.
The alert seems to have derived from information seized after the arrest of an alleged al-Qaeda leader in Pakistan last week.
US officials said the intelligence involved "extraordinary detail".
Pakistan has said that plans for attacks on targets in the US and UK were found in e-mails and documents on the computer of Ahmed Khalfan Ghailani.
The Tanzanian, wanted in connection with the 1998 US embassy bombings in East Africa, was arrested on 25 July in Pakistan.
It is not clear how significant a factor the discovery was in the American decision to issue its warning. But the information discovered in Pakistan is described as detailed and precise.
The move comes a day after New York authorities warned residents and businesses of a possible new al-Qaeda suicide attack in the city.
Homeland Security Secretary Tom Ridge said in a televised news conference that there was no indication when the attacks might be attempted, but it could be at any time in the next three months, ahead of November's presidential poll.
He said security would be increased at buildings specified in the intelligence, which included International Monetary Fund and World Bank buildings, the New York Stock Exchange, the Prudential building in Newark, New Jersey, and the Citigroup CN buildings in New York.
"This is not the usual chatter. This is multiple sources that involve extraordinary detail," he added.
NY mayor's message
Pakistani Information Minister Sheikh Ahmed Rashid told the Associated Press: "We got a few e-mails from Ghailani's computer about [plans for] attacks in the US and UK."
The BBC's Zaffar Abbas, in Islamabad, says it appears that US investigators were able to unscramble information on the computers after Pakistan passed on suspicious encrypted documents.
Pakistan has also confirmed the arrest of another man believed to be an al-Qaeda computer and communications expert, but it is not clear if this is also linked to the alleged US plot.
Meanwhile, Mr Ridge said staff and visitors to the named buildings would be warned and urged to be extra vigilant.
"Al-Qaeda wants to intimidate us," he said, adding: "Our resolve is indivisible and unyielding."
He said the US authorities understood the "preferred method of attack is car and truck bombs".
Just a few hours later, New York Mayor Michael Bloomberg urged city residents to go about their business, saying security had been stepped up at the specific targets.
The city has already been planning a massive security operation to protect the Republican National Convention in late August, when the party will nominate President George W Bush as its candidate for the November election.
Mayor Bloomberg said New Yorkers should be vigilant
"Let me assure all New Yorkers of one thing: we are deploying our full array of counter-terrorism resources," Mr Bloomberg told a news conference.
"We will spare no expense and we will take no chances."
The mayor added that on Monday morning New Yorkers should "get up... [and] go about their business and enjoy the very freedoms that the terrorists find so threatening".
Appearing alongside the mayor, New York Police Commissioner Raymond Kelly said vehicles were being searched at city bridges and tunnels, while police officers - both uniformed and undercover - were on patrol.
Mr Kelly said attacks might be carried out with car bombs or backpacks filled with explosives.
New York was the US city that suffered most casualties on 11 September 2001, with almost 2,800 people killed.