But the most tantalizing detail -- the identity of the blackmail target -- remains a secret. A judge's order bans identifying him or any witnesses in the case.
Ian Strachan, 31, and Sean McGuigan, 41, are charged with demanding $100,000 from their victim to hand over video and audio recordings containing a claim that he engaged in a sex act with another man.
Prosecutor Mark Ellison said one audio recording included an aide to the royal "apparently asserting that the member of the royal family who employed him had performed an act of oral sex on him."
Strachan claimed in a conversation recorded by police that some of the material on the tapes "was very graphic and goes into a lot of detail."
The royal was identified in court only as Witness A, and strict restrictions are in place to protect his identity. Judge Jeremy Cooke has ordered that the public be banned from the court during portions of the trial when some audiovisual evidence is played.
Buckingham Palace has refused to comment on the case, but British media have reported that the blackmail target is not a senior member of the family.
Ellison said Strachan and McGuigan were initially motivated by strong dislike of the royal aide, identified as Witness D. Early in 2007, they used a mobile phone to covertly record eight hours of audio and video footage of him.
The prosecutor said much of the footage showed the aide "drunk or under the influence of other substances." In addition to the sex claim, the footage shows Witness D apparently taking drugs - at one point allegedly using a credit card from Harrods department store to cut lines of cocaine - and making "disparaging remarks about various members of the royal family," Ellison said.
Ellison said the aide also made "allegations of impropriety as to how his employer conducted aspects of his business."
Ellison said Strachan and McGuigan tried to sell the material on the recordings to British newspapers for as much as $400,000. He said that when the newspapers declined to pay, the defendants attempted to set up a meeting with the royal to hand over the tapes in return for money.
Amid the negotiations, London police were called in by the royal and his staff.
Strachan and McGuigan were arrested Sept. 11 in a police sting operation at a London hotel. An undercover detective posing as a royal aide contacted the alleged blackmailers and arranged the meeting at the London Hilton on Park Lane.
Both men deny blackmail, which carries a maximum sentence of 14 years in prison. McGuigan told police after his arrest that "there was no question of extorting money out of people at all." McGuigan said the men's motive was to "sort Witness D out for his actions and what he did."
The prosecutor, however, said that "when you consider what they each said and did as a whole ... the evidence proves that they were both together engaged in blackmail."
Ellison told the jury that an expert who had examined the recording believed that "the most sensational audio file" - the one with the sexual allegation - "has been created by sections from different recordings being edited together."
He told the jury that the key point for them should be not "whether the material the defendants deployed was true or false, but the way it was presented ... and whether it was used to make an unwarranted demand with menaces with a view to their gain that amounted to the offense of blackmail."
The trial is expected to last a month.
By JILL LAWLESS