One of the children murdered in the Connecticut school massacre was Noah Posner, a six-year-old Jewish boy. His twin sister was also a pupil in the school, but escaped with her life.
The rabbi of Newtown, Rabbi Shaul Praver, spoke with the congregation at Edath Israel synagogue at a deeply emotional Sabbath service following the tragedy.
In condolence letter to U.S. President Barack Obama following the massacre, Prime Minister Binyamin Netanyahu drew a parallel to the acts of barbaric Arab terrorism Israel has to deal with.
"I was shocked and horrified by today's savage massacre of innocent children and adults at the Sandy Hook Elementary School in Connecticut," Netanyahu wrote.
"We in Israel have experienced such cruel acts of slaughter and we know the shock and agony they bring.
"I want to express my profound grief, and that of all the people in Israel, to the families that lost their loved ones.
"May you and the American people find the strength to overcome this unspeakable tragedy.
The horrific massacre took place Friday at a Connecticut elementary school, where a lone gunman murdered 20 children and six adults before killing himself. He had earlier shot and killed his own mother.
The murderer has been identified as Adam Lanza, 20.
Lanza shot his mother in the head at their Newtown, Connecticut, home, then went to an elementary school in the town and killed 20 children and six adults before taking his own life.
Friday morning he appears to have started his day by shooting his mother Nancy in the face. He then drove her car to nearby Sandy Hook Elementary School, armed with two automatic handguns and a semi-automatic rifle.
He then shot and killed 20 children, between five and 10 years of age. Six adults were also killed at the school. Nancy Lanza was found dead in her home.
A relative told ABC News that Adam was "obviously not well." Family friends in Newtown also described the young man as troubled and described Nancy as rigid. "[Adam] was not connected with the other kids," said Barbara Frey, who also said he was "a little bit different ... Kind of repressed."
Nancy and her husband Peter, Adam's father, divorced in 2009. Peter Lanza, who drove to northern New Jersey to talk to police and the FBI, is a vice president at GE Capital and had been a partner at global accounting giant Ernst & Young.
Israeli President Shimon Peres on Saturday called the massacre "atrocious" and "incomprehensible" and said that Israelis' hearts were with the bereaved families.
In a letter of condolence to President Barack Obama, made available to media, Peres said there was no experience that could be likened to that of parents' loss of a child.
"We stand with you today in contemplation and grief over the atrocious, incomprehensible massacre of 20 children and six adults -- educators – at Sandy Hook Elementary School," he wrote.
"No crime is more heinous than the killing of a child," he added. "Our thoughts and prayers are with you."
Earlier, Tzipi Livni, head of the newly formed centrist party Hatnuah and a former foreign minister, sent a message to US Secretary of State Hillary Clinton.
"On this sad day, all Israelis share the deep sorrow of the Americans over the loss of so many lives of women, men and innocent children," she said.