A shul-member died after suffering a suspected cardiac arrest during a synagogue service.
John Quint collapsed at Finchley United Synagogue during last Saturday's Shabbat morning prayers.
The 79-year-old was reported to have lost consciousness in the main shul building.
A synagogue member said several congregants had tried to revive Mr Quint with a defibrillator before emergency services arrived.
While members decamped to an adjoining minyan, further attempts were made to revive Mr Quint, whose brother-in-law was with him at the time.
The attempts continued for more than half an hour before he was pronounced dead.
"The general mood was shock and sombre," said an eyewitness. When people were moved from the shul, "you just didn't know what to do.
"There were hundreds of families there with their kids. It can be very hard to contain crowds."
Mr Quint's family said his death had "left a gaping hole in our lives".
The synagogue's community director Shimon Gillis said Mr Quint, who was from Hendon, "was a highly-regarded member of the Kinloss community and will be greatly missed.
"He loved the synagogue and was looking forward to sharing his 80th birthday with the community next month."
Mr Gillis added: "As soon as John displayed symptoms, first aiders, assisted by members who are doctors and consultants, with the support of London Ambulance Service, were able to give treatment.
"We would like to thank those who gave their support and expertise."
Andrew Brayam, United Synagogue health and safety manager, said the organisation had a dedicated team concentrating on its shuls.
"We have the required first aid equipment across all our synagogues, including state-of-the-art defibrillators."
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