In the week since, Israel has retaliated by bombing Gaza, from where the militants launched the attack.
Civilians have been caught up in the reprisals, as they try to flee south and thousands have died on both sides of the conflict.
Israel’s prime minister, Benjamin Netanyahu, has said the “next stage” of the country’s response is coming – with a ground offensive, supported by air and sea attacks, expected on Gaza.
Speaking on Saturday evening, Mr Sunak defended Israel’s right to defend itself, saying: “We stand with Israel, not just today, not just tomorrow, but always.”
“And I stand with you, the British Jewish community, not just today, not just tomorrow, but always.
“Am Yisrael Chai,” he added.
Am Yisrael Chai translates as ‘the people of Israel live’.
Mr Sunak said that he knew the coming days and weeks would be “very difficult”.
“No words can begin to describe the horror and barbarism unleashed in Israel a week ago,” he said. “Daughters, sons, mothers, fathers, husbands, wives, grandparents taken from people in the cruellest and most horrific way possible.
“Hundreds of people have been killed, many wounded or missing and others living through the unimaginable agony of having a loved one kidnapped and held hostage.
“British citizens were among the victims. And as we continue to learn more, I know there are families here and in Israel in deep pain and torment.
“My thoughts and my heart go out to everyone suffering in the wake of these attacks.”
‘We will do everything to protect Jewish people’
Mr Sunak also offered a message to the people of Israel.
“Britain is with you,” he said.
“What took place was an act of pure evil and Israel has every right to defend itself. We will do everything we can to support Israel in restoring the security it deserves.”
He told the British Jewish community: “I know you are hurting and reeling from these vile terrorist acts.
“At moments like this, when Jewish people are under attack in their homeland, Jewish people everywhere can feel less safe.
“We’ve seen intimidating behaviour and shameful antisemitism online and on our streets with attempts to stir up tensions.
“I say, not here.
“Not in Britain. Not in our country. Not in this century.
“We will do everything we possibly can to protect Jewish people in our country.
“And if anything is standing in the way of keeping the Jewish community safe, we will fix it.”
Labour leader Sir Keir Starmer also marked a week since the Hamas attack, while calling for safe humanitarian corridors for Palestinians in Gaza fleeing violence.
He said: “A week ago we awoke to the unimaginable and heartbreaking news of terrorist attacks on Israel from Gaza by Hamas.
“In the days that have followed, we have heard horrific stories of the murder and mutilation of men, women and children, along with the horror of hostage-taking.
“Israel has the right, indeed the duty, to defend herself and rescue these hostages. Responsibility for what has happened sits with the terrorists of Hamas; and we repeat our call for Hamas to release all hostages.
“I’ve met with members of the British Jewish community this week and told them that we stand with Israel and with them at this time.”
Sir Keir added: “I know this is a distressing and worrying time and welcome the extra funding for the CST (Community Security Trust).
“There must be zero tolerance of any increase in antisemitism or Islamophobia.
“Hamas has no interest in peace. No interest in protecting Palestinians.
“We call on all parties to act in line with international law, including allowing humanitarian access of food, water, electricity and medicines to Gaza and ensuring safe humanitarian corridors in Gaza for those fleeing violence.
“Seven days on from the darkest day in Israel’s recent history, our resolve in the face of terrorism will not falter.”