JERUSALEM: On the first Friday evening that Jerusalem restaurant Azza 40 opened without a kosher license, allowing it to serve customers on the Jewish sabbath, crowds of ultra-Orthodox Jews protested outside, threatening to smash windows and burn the place down.

"It was crazy," said Reut Cohen, 29, the restaurant's owner and head chef, recalling the events of September 2014. "The police came, the street was blocked, there were religious people yelling, swearing, even spitting at us."

It was just one of many protests, most of them peaceful, that ultra-Orthodox groups have mounted against cafes, restaurants and cinemas that open on Shabbat, the Jewish holy day. Several have been led by Jerusalem's ultra-Orthodox deputy mayor, Yitzhak Pindrus.

Azza 40 is still going strong, with Friday nights and Saturdays the busiest days of the week, despite occasional disruptions. But the pressure on businesses not to open between sunset on Friday and sunset on Saturday has increased, fuelling tension between the growing Orthodox community and those who feel religious strictures are impinging on their freedom.

Perhaps the clearest illustration of the potential fallout of what's been dubbed by local media the "Shabbat war" came this month in Tel Aviv, a city normally known for its secularism.

Work on a new railway station and track maintenance had to be suspended on a Saturday after complaints from religious parties in Prime Minister Benjamin Netanyahu's coalition.

It was a desecration of the holy day, said the chief rabbinate, which had largely ignored such work in the past but had been facing pressure in ultra-Orthodox newspapers and on social media to demand it stop.

"Shabbat is not open to negotiation and haggling, and there's no place to compromise its sanctity," said the chief rabbis in a statement explaining their position.

As a result, work was moved that weekend to Sunday, the start of Israel's week, causing traffic meltdown as the main Tel Aviv highway was partially shut down. Special buses - ironically organised on a Saturday - were laid on.

The dispute caused turmoil in Netanyahu's cabinet, with the transport minister, who supported Saturday work, on the firing line. He kept his job, and construction was quietly resumed a week later, including on Saturdays, a sign neither Netanyahu nor his ultra-Orthodox partners wanted the coalition derailed.

But while Tel Aviv was briefly shaken by the debacle, the sharp end of Shabbat tension remains Jerusalem, where the ultra-Orthodox make up a third of the population, an increase of five percentage points over the last decade, and religion is never far from any issue.