The Office star argued that the term no longer refers to its original meaning - a derogatory term for Down's syndrome sufferers.
Responding to followers who disagreed with his use of the word, he insisted: "Just to clarify for uptight people stuck in the past. The word mong means Down's Syndrome about as much as the word gay means happy. I never use the word mong to mean anything to do with Downs Syndrome."
However, comedian Richard Herring slammed Gervais on his blog, writing: "Ricky Gervais is new to Twitter and seems to have spent the first couple of weeks mainly posting pictures of himself pulling the kind of faces that school children pull to parody the disabled and calling people 'mongs'.
"I think many comics are guilty of using [the word] as convenient and humourless punchlines. I don't think any of them would do the same with the word 'n***er' or 'p**i' but they're happy to use 'mong' or 'retard' as a means of getting a laugh. And audiences will laugh at those words too and rarely even complain about them.
"If the words are upsetting some people and perpetuating a stereotype, isn't it more noble and thoughtful to just admit you might have made a mistake and stop?"
Fellow stand-ups Chris Coltrane and James W Smith also condemned Gervais for his behaviour.
However, Gervais hit back, telling his followers: "Dear fans. Don't give the haters any attention. Those people aren't really offended by the things I say - they are offended by my success."
The Life's Too Short writer was originally criticised last year for using the word to describe Susan Boyle in his stand-up show Science.
> Ricky Gervais: 'Royal wedding fans are mongs'