At an emotional memorial service at one of the former imperial capital's most famous symbols, a chamber choir sang as several hundred mourners looked on.
Outside, the bell rang 224 times, the cathedral's majestic golden dome prominent in the grey autumn sky.
Russia mourns victims of crashed Egypt plane
The ceremony was broadcast live on national television, with pictures of the victims accompanied by the sound of the bell.
Staff at the St. Isaac's Cathedral, which is a museum, said that the catastrophe had claimed the lives of one of their employees, Irina Sharova, and her 13-year-old daughter.
One of the mourners, Alla Mikhailova, said she could not stop thinking about the crash, Russia's deadliest aviation tragedy.
Russia returned 11,000 Russians from Egypt in last 24 hours -RIA
"A week has passed but I still cannot come to my senses," the 38-year-old told AFP. "I believe this, this wound will remain with us forever."
Maria Semenchuk, 50, said she had come to pray for the victims.
"That's the only thing we can do for them," she told AFP outside the cathedral.
An Airbus A-321 carrying 224 people, most of them Russian tourists, crashed in the Sinai Peninsula as it was returning from the Egyptian resort area of Sharm el-Sheikh to Saint Petersburg on October 31.
US, UK say bomb may have downed Russian jet
Most of the passengers were from Saint Petersburg and the surrounding region.
After Washington and London said they believed the Russian passenger jet might have been taken down by a bomb, Moscow on Friday halted all flights to Egypt.
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