TV personalities and a Euro MP supported the JCC London's Mitzvah Day events.
Great British Bake Off presenter Sue Perkins and Camden Mayor, Councillor Abdul Quadir, dropped into the World Peace Garden by Hampstead Heath station to help JCC volunteers put up decking, clear leaves and weeds and plant bulbs.
Clara Shapero, 11, said: "When I look at this beautiful peace garden, I will see what happens when the community works together to save an area."
Alex Helfgott, 12, observed: "It felt like everyone was trying their best to improve the community. That's how people should always work."
JCC chief executive Nick Viner said it was "a truly local initiative and a fantastic example of what volunteering can do from older and younger people, teenagers and businessmen."
In Hampstead High Street, former X Factor star Katie Waissel was among the donors to JCC volunteers who collected £675 worth of non-perishable food for the New North London Synagogue drop-in centre for destitute asylum seekers.
Posing for pictures with some of the younger volunteers, she declared herself "really pleased to do my bit for Mitzvah Day".
Other JCC projects included 40 volunteers clearing an overgrown outdoor classroom for use in science, art and environmental studies at Gospel Oak School.
The group was joined by London MEP Marina Yannakoudakis.
It also ran Singing to Seniors at Spring Grove Care Home in Finchley Road, which is close to where the new JCC for London building is being constructed.