"You may feel that, as non-Germans, this is none of our business," write the 34 celebrities, none of whom belongs to the Church of Scientology. "But ...when a modern nation demonstrates its unwillingness to protect the basic rights of a group of its citizens, and indeed, exhibits a willingness to condone and participate in their persecution, right-thinking people in other countries must speak out.
"Extremists of your party should not be permitted to believe that the rest of the world will look the other way. Not this time."
The stars, who also include the film director Oliver Stone and the novelists Mario Puzo and Gore Vidal, complain that members of Scientologists are banned from German political parties, excluded from jobs in public service, and their children are expelled from schools. "And - like the book-burning of the 1930s - your party has organised boycotts and seeks to ban performances of Tom Cruise, John Travolta, Chick Corea and any other artist who believes in Scientology.
"In the Germany of the 1930s, Hitler made religious intolerance official government policy," the letter stated. "Jews were first marginalised, then excluded from many activities, then vilified and ultimately subjected to unspeakable horrors." It added: "In the 1930s, it was the Jews. Today it is the Scientologists."
A spokesman for the International Herald Tribune in Paris said the advertisement was placed by Bertram Fields, a Los Angeles entertainment attorney who also signed the letter.
There is an element of truth in the charges, but they are not entirely accurate. The "boycott" of Tom Cruise consisted of a group of young Christian Democrats distributing leaflets outside cinemas showing his latest film, Mission: Impossible. Chick Corea was prevented from performing at a publicly funded concert because he was a Scientologist.
All parties, however, support moves to curb the activities of a group they do not accept as founded on religious principles, and a ban on Scientologists in public services has been enacted in some conservative Lander. Mr Kohl's government is now trying to extend that nationwide. Authorities also agreed to consider putting Scientology under surveillance by the anti-extremist Office for the Protection of the Constitution. Many other countries, including Britain, have taken steps to curtail the Church of Scientology.
Heber Jentzsch, president of the Church of Scientology International, said in a statement released in London that he hoped "Chancellor Kohl heeds their admonition to restore democratic principles in his country". The Chancellor himself was not prepared to respond to the criticism, however. "They don't know anything about Germany and they don't want to either," Mr Kohl said. "Otherwise they wouldn't have talked such rubbish."
Ignorance cuts both ways. Mr Kohl admitted he had not seen the letter, and asked if he planned to respond, he said: "No, I do not have any intention whatsoever of reacting. I haven't read the names of those who signed this thing." He was supported by Rudolf Scharping, parliamentary leader of the opposition Social Democrats. "This letter is unacceptable, if only because of the scandalous comparison between today's Germany and Hitler's fascism," he told the newspaper Bild.