The security measures, likened to a "ring of steel" around Windsor Castle, near London, and the procession route for the wedding, will include heavy metal barriers to deter and prevent terrorist attacks by vehicles, and sweeps of cars.
Well-wishers who want to line the streets of the procession are being warned to expect a "screening and searching" regime, including the inspection of any sleeping bags that could conceal suspicious materials.
If too many people descend on Windsor, the town will be put on lockdown, with trains no longer stopping to drop passengers off at its stations and people turned away from overcrowded procession routes, which will be lined by members of the armed forces, with regiments selected for their links to Prince Harry.
More than 100,000 people are expected to arrive at Windsor for the May 19 wedding, flying from around the world to join the festivities.
Visitors will be warned to travel early, with long waits to leave as the day's celebrations die down. Thames Valley Police will be out in force, with extra armed and unarmed officers patrolling, search dogs, a mounted section, and the National Police Air Service present to combat the "severe" threat of terrorism.
Windsor will become a virtual fortress, with each route into the town subject to careful searches, with visitors walking through scanners, expected to show the contents of their bags, and discouraged from bringing tents.
Residents will notice more CCTV cameras, extra technology to recognise number plates and "hostile vehicle mitigation barriers" to protect pedestrians from terror attack by cars or heavy duty vehicles.
Police have so far received no specific intelligence about targeted attacks, but are aware of potential protests including plans for a "royal sleepover", where campaigners plan to sleep on the streets in support of local homeless people.
Prince Harry and Ms Markle will marry at St George's Chapel, with 600 guests invited to watch the ceremony and a further 2,640 members of the public allowed to stand inside the castle walls to watch their arrival and departure.