Four radical Islamists admitted in court on Wednesday plotting to bomb the London Stock Exchange as part of a campaign of al Qaeda-inspired attacks across the British capital in the run-up to Christmas 2010.
The conspiracy included plans to post bombs to the United States Embassy and the home of London Mayor Boris Johnson.
Police foiled the plot at an early stage before firm dates were agreed or explosive devices assembled.
The plan was to cause terror, economic harm and disruption rather than injury, prosecutor Andrew Edis told London's Woolwich Crown Court.
However, their chosen method meant there was a risk people would be maimed or killed, he said.
The four, with five other men, admitted a range of terrorism offences after changing their pleas shortly before their trial had been due to begin, the Press Association reported.
The defendants, all British nationals with Bangladeshi or Pakistani backgrounds, had been inspired by al Qaeda and the late radical Muslim cleric Anwar al-Awlaki, Edis said.
Al-Awlaki, a U.S. citizen linked to al Qaeda's Yemeni branch, was killed last year in a CIA drone strike.
Undercover officers had followed two of the conspirators in November 2010 as they made observations of London landmarks including the Big Ben clocktower, parliament, Westminster Abbey and the London Eye ferris wheel.
The two men, Mohammed Chowdhury, 21, and Shah Rahman, 28, both from east London, admitted preparing for acts of terrorism by planning to plant an improvised bomb in the toilets of the London Stock Exchange.
Brothers Gurukanth Desai, 30, and Abdul Miah, 25, both from Cardiff in Wales also pleaded guilty to the same charge.
Some of the defendants had also discussed leaving home-made bombs in the toilets of pubs in Stoke, in the English midlands.
The judge told Chowdhury he could expect to receive 18-1/2 years and Rahman 17 years, although the actual time spent in jail would be shorter, around six years, taking account of time already served and parole.
The five other men, one from Cardiff and four from Stoke, admitted lesser terrorism offences including attending operational meetings and fundraising.
All will be sentenced next week.
(Writing by Tim Castle; Editing by Janet Lawrence)