The plot to which Dhiren Barot, 34, pleaded guilty, a prosecutor said, involved blowing up financial institutions in US cities as well as detonating gas cylinders and explosives in underground car parks in Britain.
The plot also involved detonating a radioactive "dirty bomb" in Britain that was intended to cause "injury, fear, terror and chaos", prosecutor Edmund Lawson told Woolwich Crown Court in southeast London.
Wearing a khaki-coloured zip-up sweater and black shirt, Barot showed no emotion when he said "I plead guilty" to one count of conspiring with other people between January 1, 2000 and August 4, 2004 to commit murder.
Mr Lawson said Barot, who sported a short beard, "has indicated that he pleads guilty on the basis that count one concerns both the US and the UK."
US officials said following the trial in London they plan to request the extradition of Berot and two other Britons to stand trial on terrorism charges in the United States.
Mr Lawson said that by pleading guilty, Barot, who will be sentenced at a date yet to be announced, "makes no admission with regard to the involvement of any of his seven co-defendants in the conspiracy".
Seven other men are due to face trial next year.
Mr Lawson asked presiding Justice Butterfield to recall that there were plans or proposals that were found by the police on a computer after the arrests of
August 2004.
"Being plans for attacks on the International Monetary Fund and World Bank buildings in Washington, the New York Stock Exchange and Citigroup buildings in New York and the Prudential buildings in Newark," he said.
"These being plans ... to carry out explosions at those premises with no warning, they were basically designed to kill as many innocent people as possible."
The "cornerstone" of the plot in Britain was a plan to blow up three limousines packed with gas cylinders and explosives in underground car parks.
The so-called "Gas Limos Project" was supplemented by "three other projects which were presented for consideration", including the "Rough Presentation for radiation or Dirty Bomb Project", he said.
Mr Lawson said the three additional projects, including the "Dirty Bomb" plan, were designed to be executed in a "synchronised, concurrent and back-to-back" way with the main Gas Limos Project.
Barot had also faced 12 other charges: one of conspiracy to commit public nuisance, seven of making a record of information for terrorist purposes and four of possessing a record of information for terrorist purposes.
The judge ordered all these charges to lie on file following his guilty plea to conspiracy to murder.