LONDON (AP) — Royalty, politicians and military chiefs are gathering at Westminster Abbey to mark half a century of Britain's sea-borne nuclear arms program — though organizers insist they are not thanking God for atomic weapons.

Prince William is set to join the service Friday in honor of Britain's nuclear-armed submarines and their crews. At least one U.K. sub carrying nuclear missiles has been on undersea patrol at all times since April 1969 — a 50-year mission titled Operation Relentless.

The abbey says the service isn't a celebration, but a recognition of the Royal Navy's commitment to "effective peacekeeping."

But peace activists have condemned the service and plan to hold a "die-in" protest outside.

Kate Hudson, secretary-general of the Campaign for Nuclear Disarmament, said the event was "morally repugnant."