The Liberal Democrat parliamentary candidate for Hampstead and Kilburn has called for calm as more than 15,000 people (and counting) signed a petition calling for him to be sacked from his party.

Maajid Nawaz, a former Islamist who now runs a leading anti-extremist organisation, caused outrage among some sectors of the Muslim community after publishing online a cartoon depicting the Prophet Muhammad.

The image – posted on his Twitter account – originated from two audience members from the LSE Atheism Society who recently appeared on a BBC show wearing T-shirts showing two cartoon figures under the title “Jesus and Mo”.

In an attempt to demonstrate that not all Muslims were offended by “bland” depictions of the Prophet Muhammad, Mr Nawaz re-posted the picture alongside the message: “This is not offensive and I’m sure God is greater than to feel threatened by it.”

In a furore that has led to multiple death threats and has forced him to contact the police, Mr Nawaz has found himself at the centre of a battleground between free speech advocates and those calling for respect of religious values.

A petition claiming he had been “disrespectful” and demanding he be sacked from his party has so far received more than 15,000 signatures.

George Galloway, MP for Bradford West, was among those condemning him, saying: “No Muslim will ever vote for the Liberal Democrats anywhere ever unless they ditch the provocateur Maajid Nawaz.”

The uproar has also prompted thousands of free-speech advocates, anti-extremist campaigners and fellow Muslims from across the world to come out in support of Mr Nawaz, a former member of the Islamist group Hizb ut-Tahrir who now runs the anti-extremist Quilliam Foundation.

Supporters include prominent atheist and scientist Prof Richard Dawkins and human rights campaigner Peter Tatchell.

Prof Dawkins tweeted: “We’ve reached a new low when you can get threats for not being offended by something.”

He added: “There really are people so in love with offence, they’re offended that you’re not offended.”

Pleading for calm on both sides, Mr Nawaz said: “I do not find it [the cartoon] offensive, so the whole debate should be about the meaning of offence, and not about whether it’s right or wrong to insult prophets, when I don’t think this is insulting.

“Some are angry that I didn’t find the cartoon featured on BBC offensive and repeated my view.

“Others are angry I am being silenced. Please let’s all calm down.”

A spokesman for the Liberal Democrats defended Mr Nawaz’s “right to express his views”.

But he urged all candidates “to be sensitive to cultural and religious feelings and to conduct debate without causing gratuitous or unnecessary offence”.