Jeremy Corbyn said his thoughts were with those whose lives were "shattered" by the September 11 attacks - but faced criticism after referring to the "wars and terror" that followed.
The Labour leader, who played a leading role in the Stop the War Coalition opposed to the military interventions in Afghanistan and Iraq, came under attack on social media after posting a tweet on the 15th anniversary of the attack.
Mr Corbyn wrote: "My thoughts are with those whose lives were shattered on 9/11/2001 - and in the wars and terror unleashed across the globe in its aftermath."
The tweet was an edited form of a slightly longer statement, released before the social media post, in which Mr Corbyn had described the attacks as a "horrific atrocity".
Twitter user Tom - @RealTomH - said Mr Corbyn: "Can't just remember the victims of 9/11 on 9/11. Is incapable of it."
Robbie Travers, who described himself as an arch-Blairite, said: "Is anyone surprised that Jeremy Corbyn is using 9/11 to advance his anti-Western ideology?"
Dan Kaszeta, a security expert, wrote: "Damn you Corbyn. I cheated death that day."
London mayor Sadiq Khan posted his own message to mark the anniversary of the terror attack, writing on Facebook: "Today marks 15 years since the 9/11 terror attacks which claimed the lives of 2,977 victims, injured thousands more and tore a hole in the lives of countless friends, families and loved ones. 67 British citizens died on 9/11, and many more victims had close ties to London.
"This was an atrocity that touched all of us who lived through it.
"Today our city stands shoulder to shoulder with our American friends as we mourn all those who died 15 years ago. I look forward to paying my respects in person when I visit New York next week."
Labour former home office minister and ex-MP Tony McNulty referred to Mr Khan's message and said: "Good solid unequivocal statement in memoriam of 9/11 - free from any weasel words about other events."
A spokesman for the Labour leader said: "As Jeremy said in his statement, the 9/11 attacks were a horrific atrocity. The statement speaks for itself."
The full statement released by Mr Corbyn said: "My thoughts are with those whose lives were shattered 15 years ago in the horrific atrocity on 9/11 - and in the wars and terror unleashed across the globe in its aftermath."