Two rabbis were said to be among a group of potential targets for a gang of nine terror suspects, a court has heard.
Other possible targets for the alleged bomb plot included the London Stock Exchange, the US Embassy, Big Ben and the Palace of Westminster.
Details of who the two rabbis are, and their synagogues, cannot be reported for legal reasons.
The gang appeared at Westminster Magistrates' Court on Monday and were remanded in custody. They
will appear at the Old Bailey on January 14.
Anti-terror officers arrested the men in dawn raids in London, Cardiff and Stoke, last week.
Following the arrests police found a handwritten note containing the full contact details, including name, address and postcode, of six individuals. It was this list which included the rabbis' details.
The Community Security Trust (CST) said it had been informed of the possible threat to the rabbis before the court appearances, and was working with police.
In a statement, CST said: "We were alerted by police and together we briefed the rabbis, and also the security officers at their synagogues.
"We are grateful to the police for their actions, and regard this case as another unfortunate example as to why our community needs to have rigorous security procedures
The court heard that a covert surveillance operation had been carried out by police and MI5. It included the suspects being followed for months and their conversations being recorded.
Police observed the defendants, aged between 19 and 28, allegedly making a reconnaissance trip from Trafalgar Square, down Whitehall to Westminster Bridge, where they apparently studied Big Ben.
Documents were also discovered by police including what was described as an al-Qaeda newsletter and details of how to make bombs and "participate in jihad".
The nine men are of Bangladeshi origin and all face two charges: conspiring to cause an explosion or explosions "of a nature likely to endanger life or cause serious injury to property in the UK" between October 1 and December 20 2010, and a charge of "engaging in conduct in preparation for acts of terrorism" between the same dates.
Anwar Al-Awlaki, a Yemeni preacher who promotes violent jihad, is alleged to be the inspiration for the plot, the court was told. The operation to arrest the men was the most high-profile anti-terror raid in Britain since April last year, when a dozen people were detained across northern England.
They were all later were released without charge.