The commitment came after one radical group, Muslims Against Crusades, made a formal bid for permission to demonstrate outside Westminster Abbey on the big day.
Senior officers rejected the application but admitted they have no powers to ban a stationary protest elsewhere in central London.
The group's US-registered website has a clock counting down the hours until the wedding on April 29 and includes a mocked-up photo of Prince Harry wearing a swastika beret.
"We strongly advise Prince William... to withdraw from the crusader British military and give up all affiliation to the tyrannical British Empire," the group's website warns.
"We promise that should they refuse, then the day which the nation has been dreaming of for so long will become a nightmare..."