AMNA NAWAZ: In a day of political upheaval, Britain's Prime Minister Rishi Sunak brought back former Prime Minister David Cameron as his foreign secretary.

And Sunak fired Home Secretary Suella Braverman after critics claimed her remarks contributed to violence during pro-Palestinian marches this weekend.

The British government is also pledging to crack down on antisemitism, which has surged across Europe since the October 7 Hamas attacks in Israel.

From London, special correspondent Malcolm Brabant reports.

(CHANTING) MALCOLM BRABANT: According to police, 300,000 marched 105 years to the day the First World War ended, Armistice Day.

PROTESTERS: Cease-fire now!

Cease-fire now!

TAIBA RAJAB, Protester: There's no need to eradicate the whole of Palestine.

And basically what they have done is, they're just ethnic cleansing and pushing everyone out.

And that's not OK. NAZ AKCHAR, Protester: I can't see children being killed, a genocide that's going on in Palestine, the war crimes that have been committed by the IDF.

And everyone, the -- all the governments are complicit in this.

SAM SIDDIQI, Protester: To be honest, I'm just extremely disappointed in most of the governments around the world.

I think the reaction to what's happening in Gaza is pretty disgusting.

It's not a case of, if you are looking negatively at the Israeli state, I don't think you're antisemitic.

MALCOLM BRABANT: This is the biggest pro-Palestinian demonstration since October 7.

Britain's home secretary wanted London's police chief to ban it and lambasted him when he refused to do so.

The police argued that the threat of trouble wasn't serious enough to warrant a ban.

And that judgment call is now going to be put to the test.

Clashes involving right-wing nationalists threatened to disrupt the traditional salute to Britain's war dead.

The nationalists said they wanted to defend the Cenotaph from Palestinian supporters and were furious at being thwarted by police.

PROTESTERS: You're not English.

You're not English.

You're not English anymore.

MALCOLM BRABANT: The two-minute silence of the 11th hour was observed peacefully beneath the statue of Winston Churchill, as a handful of Britons signalled their support for hostages held by Hamas.

Organizer Mark Birbeck: MARK BIRBECK, Pro-Israel Advocate: We have families in this country where the kids are unable to wear their signs of their religion to school.

How does that honor the memory of the people who fought for freedom?

PROTESTER: From the river to the sea!

PROTESTERS: Palestine will be free!

MALCOLM BRABANT: The government is considering outlawing this chant because it implies wiping Israel off the map.

The police say they're looking to arrest this group.

PROTESTER (through translator): Khaybar, Khaybar, O Jews, the army of the Muhammad will return.

MALCOLM BRABANT: The antisemitic chant threatens Jews with the return of Mohammed's army.

Police are also hunting a group who intimidated worshipers outside a London synagogue.

But the police and some senior government ministers blame the home secretary, Suella Braverman, for whipping up hatred before the demonstration.

SUELLA BRAVERMAN, Former British Home Secretary: Thousands of people take to the streets following the massacre of Jewish people, the single largest loss of Jewish life since the Holocaust, chanting for the erasure of Israel from the map.

To my mind, there's only one way to describe those marches.

They are hate marches.

MALCOLM BRABANT: Although Prime Minister Rishi Sunak sacked Braverman for her strident language, protecting Britain's Jewish community is paramount.

RISHI SUNAK, British Prime Minister: We will not stand for the hatred and antisemitism we have seen on our streets.

PARLIAMENT MEMBERS: Hear, hear.

RISHI SUNAK: It sickens me to think that British Jews are looking over their shoulder in this country.

PARLIAMENT MEMBERS: Hear, hear.

MALCOLM BRABANT: According to the European Union, antisemitism has not been this bad since the 1930s.

In Paris, Stars of David stenciled on buildings resurrected memories of Nazi tactics to identify Jews.

MARIE, Jewish Resident of Paris (through translator): I'm crying because I'm going to again feel the hatred that was there when we were kids.

I don't understand it.

MALCOLM BRABANT: In Germany, the government has warned that antisemitic attacks risk transporting the country back to its most horrific times.

In Denmark, Prime Minister Mette Frederiksen led last week's commemoration of Kristallnacht, when, in 1938, the Nazis launched a pogrom against Jews across Germany and Austria.

After the torchlit procession, Denmark's chief rabbi was assaulted and now wants to meet his assailant to try to prevent further attacks on the nation's Jews.

AVISHAY GAZIEL, Jewish Community Leader: It's actually more difficult to be Jewish in Denmark in 2023 than it was in 1943.

MALCOLM BRABANT: Jewish activist Avishay Gaziel: AVISHAY GAZIEL: And I know it sounds maybe a bit bizarre, but, in 1943, the enemy was visible.

The faith of a Jew was actually much more predictable.

Now we are in an unknown level of risk.

MALCOLM BRABANT: Gaziel is referring to incidents like this encounter on Kristallnacht between Denmark's Foreign Minister Lars Lokke Rasmussen and a Hamas supporter.

WOMAN (through translator): It's my people who are getting killed.

LARS LOKKE RASMUSSEN, Danish Foreign Minister (through translator): But aren't you also very angry about the terror attack Hamas carried out on October 7?

WOMAN (through translator): No, I'm really not.

I am very happy that Hamas made that decision ON October 7.

We have been in a (EXPLETIVE DELETED) since 1948.

LARS LOKKE RASMUSSEN (through translator): This is where I simply don't understand you.

Well, anyway, have a nice evening.

MALCOLM BRABANT: Adding to the tension is a video about Hamas' hostages by a Danish Palestinian influencer called Armani (ph).

"There are still people down there who have not been done," he says.

"But 250," he says, "that's not too bad."

"OK," he says, "joking aside, condolences for all those lives that have not yet been taken."

What do you fear is going to happen?

AVISHAY GAZIEL: I cannot see how this -- all this hatred we see around in demonstrations, how it channels itself to anything else than violence.

PROTESTER: Killers.

Death to all the Jews!

AVISHAY GAZIEL: I honestly cannot see how we tamp it down.

MALCOLM BRABANT: Saturday's demonstration passed the headquarters of Britain's secret intelligence service MI6, whose agents are trying to prevent the fury generated by the Gaza conflict from turning deadly.

Britain's current terrorism threat is classified as substantial, which means an attack is likely.

For the "PBS NewsHour," I'm Malcolm Brabant in London.