It is perhaps the last taboo in Hollywood but the carnage in Gaza is prompting an increasing number of artists and celebrities to do the unthinkable: criticise Israel.

Figures as varied as Rihanna, Jonathan Demme and Selena Gomez have broken the cardinal rule, and risked giving their publicists cardiac arrest, by publicly expressing solidarity with the Palestinians.

The insurrections have triggered a backlash from industry heavyweights such as Jon Voight, Joan Rivers and Howard Stern, verbal volleys in interviews and social media which are Tinseltown's version of civil strife.

"My name is Jon Voight and I am more than angry," the Midnight Cowboy star wrote in an open letter the Hollywood Reporter at the weekend. "I am heartsick that people like Penélope Cruz and Javier Bardem could incite anti-Semitism all over the world and are oblivious to the damage they have caused."

The actor was responding to a letter from more than a hundred Spanish artists, including Cruz and Bardem, accusing Israel of "genocide" in its assault against Hamas militants.

About 1,800 Palestinians, most of them civilians, have been killed and more than 9,000 injured in the past four weeks, according to health officials in Gaza. Israel has lost two civilians and 64 soldiers. A Thai migrant worker was also been killed.

Hollywood has close ties to Israel – a cause which tends to unite prominent, liberal Jews such as Steven Spielberg, Jeffrey Katzenberg and Barbra Streisand as well as conservative non-Jews such as Voight.

The bombing of UN-run schools and rising civilian casualties in Gaza however has emboldened diverse figures to speak out – only in some cases to swiftly retreat.

The actors Mark Ruffalo and Wallace Shawn, Pink Floyd's Roger Waters and the director Jonathan Demme, have experienced jeers since taking a stand.

"I don't see this as being politics or statehood for Palestine or Hamas," Demme, who directed Silence of the Lambs, told AP. "I think it's about taking innocent lives and the destruction of a culture. I've never been ashamed of my pacifist point of view of things at any time since I became a card-carrying hippie back in the '60s."

The Daily Show host Jon Stewart also waded in, choosing his words carefully. "Just merely mentioning Israel or questioning in any way the effectiveness or humanity of Israel's policies is not the same thing as being pro-Hamas."

Two weeks ago Rihanna tweeted the hashtag #FreePalestine to her 36 million followers only to delete it eight minutes later, amid a surge of critical responses, saying it was an accident. A source close to the singer told reporters she was "pro peace" and not taking sides.

Cruz and Bardem also partially retreated, issuing statements to clarify that they had great respect for the people of Israel, mourned their losses and yearned for peace in the region.

Rivers, a veteran comedian and staunch Israel supporter, mocked Gomez, 22, for expressing solidarity with Palestinians. "Where did she go to college? Ask her if she knows how to spell 'Palestinian'".

Russell Brand and Sean Hannity exchanged insults, with the British comedian labelling the Fox anchor intolerant and a terrorist, and Hannity calling Brand "Mr Katy Perry" and a D-list actor.

Much of the entertainment industry has remained silent – unusual given otherwise voluble opinions about gay marriage, climate change and south Sudan – and observed the Hollywood Reporter's dictum: "Rule 1: Talk about anything political in Hollywood ... except Gaza."

Passions are running so high, however, that even silence from the likes of Spielberg, Streisand and Katzenberg is now considered a statement of sorts.

"Some of the biggest, most well-known supporters of Israel have been the least vocal," noted the Jewish Journal. "Eager to support Israel in times of quiet, they prefer to keep a low profile when things heat up."