Noam Sagi, 51, said his mother Ada Sagi was grabbed from Kibbutz Nir Oz on October 7. She is among the 220 hostages held by Hamas.
Mr Sagi addressed a vigil at JW3 (Jewish Community Centre) in North London.
The vigil included a table of 220 empty seats each with the name and face of a hostage.
Mr Sagi, who was born in Kibbutz Nir Oz and lives in London, recounted his mother’s final call to his sister.
He said: “My mum called my sister and then my sister called me later … woke me up on Saturday morning here, very distressed, saying she their mother saw Hamas terrorists outside her house.”
Media footage showed a militant outside the home. “At six o’clock in the evening the [Israeli] army came and found bloodstains.
“She’s not on the dead list, she’s not on the injured list, she’s not on the rescue list and now it is day 21 and we still haven’t heard anything.”
Mr Sagi said: “When I saw the pictures it broke my heart; the second thing that broke my heart is people celebrating on the streets of London.
“What they celebrated is my pain, my personal pain. My mum is kidnapped and people are sitting bumping on cars, celebrating this.”
He hopes Ada, an ex-Arabic and Hebrew teacher, is communicating with her captors to allow her escape.
Raymond Simonson, 50, chief executive of JW3, said: “I’ve never seen tensions so high between Muslims and Jews in the UK. We’ve seen really awful anti-Semitism.”
