The following correction was printed in the Guardian's Corrections and clarifications column, Wednesday 28 April 2010

The article below said that Islam's prophet has featured twice in the series: this year, and in 2006. In fact, Muhammad appeared in 2001, too about a threat against makers of the satirical cartoon series South Park.

They have depicted the Queen blowing her brains out after a failed attempt by the British army to reinvade America, Saddam Hussein as Satan's gay lover, and Jesus as a trigger-happy superhero. Mormons, Scientologists, Catholics, Jews, politicians and film stars have all been skewered on the razor-sharp wit of South Park.

Now the caustic animated satire appears to have reached its limits within the confines of mainstream US television. Fans and pundits alike were taken aback last night when an episode featuring the prophet Muhammad purportedly dressed in a bear costume had bleeps and "Censored" blocks slapped liberally throughout to remove all audio and visual reference to the prophet.

The censorship followed a warning from a New York-based group of extremist Muslim converts that could be construed as a death threat. The group, through its website Revolutionmuslim.com, had reacted to last week's episode of South Park which first depicted Muhammad dressed as a bear by saying its originators, Matt Stone and Trey Parker, "will probably wind up like Theo van Gogh".

To underline the point, the website carried a picture of Van Gogh, the Dutch film-maker killed in 2004 after he made a documentary on the abuse of women in Muslim countries, with his throat cut and a knife in his chest. They also listed the New York headquarters of Comedy Central, the cable television channel that broadcasts the show, and South Park's production company, adding: "You can pay them a visit at these addresses."

In the aftermath of the show's censorship, the chatrooms on South Park's website hummed with the indignation of its fans. "Do you seriously think that will appease the extremists from more terrorism?" one wrote.

As controversy raged, Comedy Central confessed that it was responsible for the cuts. "I can't go into the thinking behind it, but I can confirm it was Comedy Central that inserted the bleeps and not South Park," a spokesman for the station said.

The apportioning of responsibility will come as a relief to devotees of the animation, who have grown accustomed to it treating nothing and nobody as sacred.

The Muhammad furore began last week in the show's 200th episode when the creators introduced the character as a riff on censorship. The joke was that Muhammad was dressed as a bear because he could not be shown as a cartoon in the wake of death threats made against Danish cartoonists by Islamist extremists, who see any depiction of Muhammad as a gross insult to their religion.

In the storyline, the prophet was brought into the show on the demand of previous victims of its satire, led by Tom Cruise, who believed that he could make them immune to further ridiculing. In this week's episode, the bear costume was unzipped to reveal that Santa Claus, not Muhammad, had been inside all along.

This was the second time that South Park had taken on the prophet as a subject. In 2006, soon after violent protests erupted against the Danish cartoons, Stone and Parker produced a double episode that featured the prophet and was also censored by Comedy Central.

In that case, however, there was no pickup from Islamist groups. The latest controversy has cast the light on Revolution Muslim, a group of probably fewer than 10 extremists based in New York who hand out leaflets outside the moderate 96th-street mosque.

It was founded by Yousef al-Khattab, a former secular Jew born Joseph Cohen who ran a bicycle pedicab in New York until he relocated to Morocco. He is shown in one photograph on the site carrying a machete with what look like suicide bombs strapped around his waist. The group has a record of making extremist statements, which it tempers with disclaimers that it is nonviolent to avoid legal trouble. One member, Younus Abdullah Muhammad, told CNN last year that the 9/11 attacks had been justified.

At other times, its website has called for wrath to fall "on the Jewish occupiers of Palestine. Please throw liquid drain cleaner in their faces."

"Many groups would be afraid to act like this for fear of arrest, but they clearly don't care," said Yehudit Barsky of the American Jewish Committee, which monitors the group.

Revolution Muslim put out a long statement in which it insisted that it was in favour of a rational dialogue with non-Muslims. But it accused South Park of having mocked the prophet, and cited Islamic scholars who ruled that "whoever curses the messenger of Allah must be killed".

Abdullah Muhammad, 30, defended the web posting by his group to Reuters.

"How is that a threat?" he told the news agency. "Showing a case study right there of what happened to another individual who conducted himself in a very similar manner? It's just evidence."

It is not clear whether Parker and Stone will feel the need to show they are unbowed by returning to the Muhammad theme next week. They are famous for putting the show together at the last minute to make it sharply topical.

But the controversy could deepen. Fans of the show on Wednesday posted in chatrooms on South Park's websites several Muhammad cartoons that would be far more offensive to Muslims than the animation itself. They included a drawing of Muhammad crying in a baby's nappy and saying: "Stop it! You hurt my feelings.""

Ten top shockers

A history of cartoon excess

1 The Passion of the Jew

In 2004, the writers took aim at Mel Gibson's The Passion for a perceived antisemitic portrayal of the crucifixion. Featuring torture, goose-stepping and German chanting.

2 Stupid Spoiled Whore Video Playlist

Paris Hilton is the target; her dog kills itself and the heiress's supposed vacuity is ridiculed in catchy songs.

3 Hell on Earth

Satan throws a Halloween costume party; Steve Irwin arrives with a stingray hanging from his chest.

4 With Apologies to Jesse Jackson

Randy Marsh causes a race storm and apologises – by kissing Jackson's ass.

5 The Ring

Writer Trey Parker forays into teen chastity; it weaves in a foul-mouthed, sadistic Mickey Mouse.

6 Osama Bin Laden has Farty Pants

Soon after 9/11, Cartman and Co go to Afghanistan to find Osama bin Laden, and stumble upon his flatulence.

7 Bloody Mary

Attack on the Catholic church caused a ban by US networks. The title featured the Virgin Mary bleeding from her rectum.

8 It Hits the Fan

Onscreen counter of how many times "shit" is said: once every eight seconds.

9 The Snuke

Parody of TV show 24 in which the British are terrorists. It ends with the Queen shooting herself in the mouth.

10 Trapped in the Closet

Aimed at Scientology and celebrity adherents. Isaac Hayes, voice of Chef, asked to leave shortly afterwards.

Aidan Jones

• This list was amended on 23 April 2010. The original listed The Passion of the Jew, Stupid Spoiled Whore Video Playlist, and Suicide of the Queen. This has been corrected.