The idea that Europeans and Russian opponents of the Kremlin are sexual deviants with a taste for pedophilia is a strange but recurring theme in Russian propaganda. The Russian ex-wife of a Norwegian man gained wide attention in state media, for example, with fabricated claims, made after she lost a child custody battle in Norway, that her former husband dressed up their 4-year-old son in a “Putin costume” and raped him.

Foes of the Kremlin have sometimes picked up the same ugly club and used it to beat Mr. Putin, as did Alexander V. Litvinenko, a former K.G.B. agent who died in London in 2006 from poisoning by a highly toxic radioactive isotope. Four months before his death, which a British inquiry ruled was probably state-sponsored murder approved by Mr. Putin, Mr. Litvinenko published an article that, without any evidence, asserted that the Russian president was himself a pedophile.

Mr. Bukovsky, who was a close friend of Mr. Litvinenko, said he had strongly urged him not to publish. “I was very angry with him,” Mr. Bukovsky recalled, noting that in many ways Mr. Litvinenko, despite his ferocious hostility toward the Kremlin, still had the mind-set of a security officer and “could not understand the difference between truth and operational information.”

On the “dark web,” an area of the internet that requires special software and authorization codes to enter, suspected Russian hackers openly offer to plant evidence of pedophilia as a way to destroy an enemy.

“I’ll do anything for money,” promised an advertisement placed by a hacker who offered to ruin “your opponents, business or private persons you don’t like. I can ruin them financially and or get them arrested, whatever you like.” Boasting that it was possible to destroy both individuals and businesses, the hacker added, “If you want someone to get known as a child porn user, no problem.” He gave a price, denominated in Bitcoins, of around $600 per job.

Paulo Shakarian, the chief executive officer of IntelliSpyre and the director of the Cyber-Socio Intelligent Systems Laboratory at Arizona State University, said his team had analyzed the advertisement and concluded that it was probably posted by a Russian (or at least a Russian-speaking) hacker. He said the price was in the normal range of what hackers demand for character assassination.

No matter what the court in Britain decides, Mr. Bukovsky has already had his reputation — and, by association, that of other Kremlin’s critics — trashed in Russia. Russian state television, in a report on the case, described the dissident as “a lover of child porn.”

Mr. Bukovsky complained that European countries that expect clarity and follow rigid procedures easily fall prey to the dirty tricks of a regime that excels in hiding its tracks and creating confusion. “They are very good at using the West against the West,” he said.