The Chief Rabbi led the tributes to veteran Zionist Arieh Handler, who died in Israel last month, aged 95, at a memorial evening at St John's Wood Synagogue on Sunday.
More than 300 people heard Lord Sacks recall Mr Handler's boyish enthusiasm and inspirational qualities.
He also noted his involvement in seminal events - he was present at the declaration of the state of Israel and assisted in the rescue of Jews from the Nazis on the Kindertransport.
Aviv Handler remembered his grandfather as a kind, gentle and loving family man who was modest about his accomplishments. The tribute, also including a video presentation, was organised by Bnei Akiva and Bachad-Friends of Bnei Akiva, two organisations Mr Handler had supported throughout his life.
Movement leader Michael Rainsbury said: "The success of Bnei Akiva over the last 70 years can be attributed to his incredible achievements and every member of Bnei Akiva looks to him as an inspirational role model.
"We commit to ensuring that Arieh's great legacy will never be forgotten".