For months, Prime Minister Keir Starmer’s Labour government has been under intense pressure from scores of its own members to demonstrate more robust support for Palestinians, especially in Gaza.

But a terrorist attack at a synagogue on Thursday in Manchester, England, which led to the deaths of two people during prayer services on Yom Kippur, exposed a competing political reality for Mr. Starmer: the growing anger among Jews in Britain who believe his government is doing far too little to protect them from hate and antisemitism.

The police have said the synagogue attack was carried out by Jihad al-Shamie, a British citizen of Syrian descent, and influenced by what they described as “extreme Islamist ideology.”

The anger at Mr. Starmer’s government was especially evident during a vigil on Friday for victims of the terrorist attack. David Lammy, the deputy prime minister, was booed off the stage by a rain-soaked crowd who angrily chanted, “Shame on you,” and loudly demanded that the Starmer government stop pro-Palestinian marches, saying they instill fear in Jewish residents.

Mr. Lammy expressed his condolences, but made little progress in calming the crowd. He pledged to “stand against all those who would minimize or coddle or obfuscate on anti-Jewish hate.” But he said nothing about banning the pro-Palestinian marches, even as members of the crowd continued to angrily call for it.

Tony Travers, a politics professor at the London School of Economics, called that demand “impossible,” but said it underscored the difficult position for Mr. Lammy, Mr. Starmer and other members of the government.

“This is absolutely insoluble,” he said. “Banning marches is not very British, certainly not very democratic. On the other hand, with the marches themselves, behaving in a sensitive, emotionally, empathetic way, is what’s missing.”

On Sunday, there will be a separate rally in Trafalgar Square in London honoring those taken hostage during the Oct. 7, 2023, Hamas-led attack and calling for the group to release the remaining hostages. The two-year anniversary of the attacks is on Tuesday.

The potential for increased attention on the demands of Jewish residents of Britain is a raw, political challenge for Mr. Starmer, who rose to power in the Labour Party by pledging to rid its ranks of antisemitism that had flourished under his predecessor Jeremy Corbyn.

Eager to repair the relationship with Britain’s nearly 300,000 Jewish voters when he was elected leader in 2020, Mr. Starmer said in his acceptance speech that antisemitism was a “stain” on his party and vowed to “tear out this poison by its roots.”

But five years later, the pressure, and the politics, has shifted.

There are about four million Muslims in Britain now, and the two-year war between Israel and Hamas has galvanized them as the death toll in Gaza from Israel’s bombardment has increased to more than 64,000, according to local officials, whose figures do not distinguish between civilians and combatants.

At the annual Labour Party conference a week ago, delegates voted to approve a nonbinding, pro-Palestinian resolution demanding an end to all arms sales to Israel and calling on the Starmer government to “employ all means reasonably available to it to prevent the commission of a genocide in Gaza.”

The delegates rejected what they called a weaker resolution that would have requested only a partial ban on arms sales.

Pressure from Muslims and left-wing allies in the Labour Party now regularly spill onto the streets of London and other British cities. Pro-Palestinian marchers regularly display banners declaring, “Free Palestine.”

That activism helped persuade Mr. Starmer to formally recognize a Palestinian state last month, a step that many pro-Palestinian activists have said was too slow in coming. More than 250 members of Parliament from nine parties, including Labour, had signed a letter urging Britain to recognize Palestinian statehood.

Christian Wakeford, a Labour member of Parliament from Manchester, said he had seen repeated pro-Palestinian protests outside his office. Anne Goldstone, 66, who lives near the synagogue that was attacked, recalled an incident a few weeks ago when a woman drove past Jewish businesses yelling, “Free Palestine.”

“Honestly, for me, the overwhelming thing is, can we have a little bit of support from the government?” Ms. Goldstone said on Friday at a Jewish bakery in Manchester that was being guarded by several police officers.

“Can they recognize that there’s an issue, and can they do something about it?” she said. “Can they help us? And I don’t mean just empty words that we support the community, but just to do something.”

Finding something that is acceptable and politically possible may be difficult for Mr. Starmer over the next weeks and months.

There is little evidence that the very loud activism of the pro-Palestinian movement is winding down, even as a peace plan proposed by President Trump appears to have gained support of both Israel and, possibly, Hamas. Past efforts to broker an end to the war and a return of the hostages have broken down.

A pro-Palestinian protest took place in Manchester on Saturday, with speakers addressing a rally outside the city’s cathedral while onlookers held Palestinian flags, before marching with chants of, “Stop bombing Gaza now.”

The demonstration, which was organized by groups including Greater Manchester Friends of Palestine, the Muslim Association of Britain and Jewish Action for Palestine, was met by a small group of counterprotesters.

Mr. Travers, the politics professor, said it was not difficult to understand why the pro-Palestinian marches, and the reaction to them, will continue.

“It’s a cause,” he said of the marches. “But because it’s coming against the backdrop of terrorism of the kind we saw in Manchester on Thursday, it doesn’t appear very sensitive, because it isn’t. That’s the trouble.”

Lizzie Dearden contributed reporting.