Attending a prayer service at Finchley United Synagogue in north London on Monday, the prime minister condemned the “barbaric acts” committed by Hamas.
He said: “Perhaps above all, I wanted to come here tonight to stand with you in solidarity in Israel’s hour of need.
“As the prime minister of this country, I am unequivocal. The people who support Hamas are fully responsible for this appalling attack.
“They are not militants. They are not freedom fighters. They are terrorists.
“Their barbaric acts, are acts of evil. There is no other word to describe what we have seen.
“Teenagers at a festival of peace gunned down in cold blood.
“Innocent men, women and children abducted, raped, slaughtered even a Holocaust survivor taken away as a captive. And much of the sickening evidence posted online.
“There are not two sides to these events. There is no question of balance.
“I stand with Israel. We stand with Israel. The United Kingdom stands with Israel against this terrorism today, tomorrow, and always.”
Israel formally declared war on Sunday, with prime minister Benjamin Netanyahu declaring a day later that his country’s offensive against Hamas in the Gaza Strip has “only started”.
Hamas claims it launched its attack in response to the decades of violence faced from the Israeli government.
The hostilities have so far have killed around 900 people in Israel and more than 680 people in Gaza, according to authorities on each side.
On Monday evening, hundreds of pro-Palestinian protesters gathered outside the Israeli embassy in Kensington, London.
Reports suggested fireworks were let off, flares were lit and chants of “Israel is a terrorist state”, “Free Palestine” and “Allahu akbar” rang out.
According to the United Nations, more than 123,000 Palestinians have fled their homes in Gaza, where families were already struggling under a 15-year Israeli blockade.