The young trees at the top end of Canfield Gardens, near Finchley Road, were snapped or had branches ripped off them between January 22 and 23.
Ham&High reader Tony Larsen said he was on his way to Finchley Road station last Thursday when he noticed a woman standing by the broken trees “looking a bit sad”.
He said: “Someone just snatched three or four of them in that kind of mindless violence that really riles you up.”
Mr Larsen said Camden Council had planted the saplings in December in gaps where old trees had been taken down.
Peter Symonds, former chair of CRASH (Combined Residents’ Association of South Hampstead), said they had begun showing signs of budding, but were deliberately broken off at the point where they were branching.
He added: “We were always agitating to have more trees planted in the area so when these trees were put in we were absolutely delighted.
“It was absolutely devastating to see that four trees should be so indiscriminately targeted.”
He said Canfield Gardens was a “rat run” between Finchley Road and West End Lane, and the area where the trees were damaged is on the C11 bus route.
He added: “It’s a particularly toxic street so to have these new trees was a joy to all of us.
“Everybody is outraged at the sight of these trees being killed off the way it was. It was just mindless vandalism really.
“It’s very hard to understand why anybody would do such a thing but there you go, it’s life today.”
Camden Council removed the damaged saplings on Tuesday (January 28) but the authority has pledged to replant them.
A spokesperson said: “The council will plant new trees in these locations before the planting season ends in March, in line with our Camden-wide efforts to increase the number of street trees providing much-needed shade, greenery, and new wildlife habitat.
“Such vandalism is unacceptable. If anyone has details about who vandalised the trees, please report it to the council.”