South Park is not known as a show that shies away from controversy, but last night its creators appeared to bow to threats of violence from a US Muslim group by censoring a typically irreverent episode about religious leaders, including Muhammad.

Wednesday night's show was labelled with the word "Censored" after the words Prophet Muhammad were beeped out during broadcast and images of the prophet in a bear outfit were substituted with ones of Santa Claus in the same costume.

But whether the changes were intended to acquiesce to the threats, or make fun of them, was not immediately clear. In the first part of the 200th episode screened last week, Muhammad appeared several times inside a bear suit, while the leaders of other religions were also depicted irreverently, including a scene which showed Buddha snorting drugs.

Before last night's show was aired, Islamists warned its creators, Matt Stone and Trey Parker, they could face severe retribution for repeating the depiction of Muhammad in costume.

A posting on the website of the US-based group, Revolution Muslim, warned Stone and Parker they would "probably wind up like Theo van Gogh" if the images were broadcast.

It posted a graphic photo of Van Gogh, a Dutch filmmaker who was stabbed to death in 2004 by an Islamic militant over a movie he had made that accused Islam of condoning violence against women. The post, which has since been removed, also included a link to a news article which contained details of a house in Colorado that the writers are thought to co-own and listed the addresses of Comedy Central's New York office and the show's production office in California.

Written under the name Abu Talhah al-Amrikee, the post said: "We have to warn Matt and Trey that what they are doing is stupid and they will probably wind up like Theo van Gogh for airing this show. This is not a threat, but a warning of the reality of what will likely happen to them."

Amrikee later said the posting was not an incitement to violence but had been published to raise awareness of the issue and to prevent it happening again.

Asked if Parker and Stone should feel threatened by it, he said "they should feel threatened by what they did". He said he was disappointed that commentary on the posting had focused on the potential danger to the producers, but admitted, "I could shoulder some blame".

It is not the first time Stone and Parker have faced calls to censor their work. In 2006, Comedy Central prevented Stone and Parker from showing an image of Muhammad in an episode designed to comment on the Danish Prophet Muhammad cartoon, which sparked worldwide protests. The majority of Muslims consider any physical representation of their prophet to be blasphemous.

Comedy Central declined to comment on the latest controversy.