Sirens blared across Israel as the country came to a standstill in an annual ritual honouring the six million Jews murdered during the Holocaust.

Pedestrians halted and drivers got out their cars as people bowed their heads in memory of the victims of the Nazi genocide.

Ceremonies were planned throughout the day at Israel's national Holocaust memorial, parliament and elsewhere.

Israel was founded in 1948 as a sanctuary for Jews in the wake of the Holocaust. It is home to about 165,000 survivors - a dwindling population widely honoured but struggling with poverty.

Ushering in Holocaust memorial day at Yad Vashem, Israel's memorial, Prime Minister Naftali Bennett late on Wednesday called on the world to stop comparing the Holocaust to other events in history.

He spoke after the presidents of Ukraine and Russia drew parallels between their ongoing war and the genocide during the Second World War.

"As the years go by, there is more and more discourse in the world that compares other difficult events to the Holocaust, but no," he said.

"No event in history, cruel as it may have been, is comparable to the extermination of Europe's Jews by the Nazis and their collaborators."

Mr Bennett also warned the country against allowing its deep differences to tear the nation apart.

His speech, on one of Israel's most solemn days of the year, came in a personal context.

On Tuesday, his family got a letter with a live bullet and death threat.

Israeli authorities tightened security around the premier and his family and were investigating.

"My brothers and sisters, we cannot - we simply cannot - allow the same dangerous gene of factionalism (to) dismantle Israel from within," Mr Bennett said.

Israel goes to great lengths to memorialise Holocaust victims and make heroes of survivors.

Restaurants and entertainment venues stay shut on Holocaust memorial day, radios play sombre music and TV stations devote their programming to documentaries and other Holocaust-related material.

For them, challenges loom.

This year's ceremony comes as Israel and much of the world emerges from the coronavirus pandemic, which confronted Holocaust survivors in particular with increased health risks as well as widespread loneliness and despair.

Additionally, about a third of Israel's Holocaust survivors live below the poverty line, with many sustained by government stipends and donations, according to a group that represents survivors.

Despite their experience and widespread education programs, antisemitism rose worldwide during the pandemic, according to a report released on Wednesday.

In addition to speeches by Mr Bennett, Israeli President Isaac Herzog and others, Wednesday's ceremony featured survivors lighting six torches.

The speaker of Germany's parliament, Baerbel Bas, was a special guest.