The 28-year-old said in a series of interviews that he took fighters "out of the game" during his 20-week posting.
Lindsey German, convenor of the Stop The War Coalition, said the comments were "crass" and asked how the Prince knew those he killed were members of the Taliban.
"In recent months many civilians have been killed by air strikes. This arrogant and insensitive attitude to killing Afghans, whoever they are, is hardly likely to win hearts and minds - a supposed aim of the war.
The Stop The War Coalition is finalising details of events next month to mark the 10th anniversary of the huge protests against going to war in Iraq.