Security in Windsor for royal wedding

Authorities are beefing up security in Windsor for Prince Harry's wedding on Saturday

Security for Saturday's royal wedding one of largest ever security operations in Windsor.

Thames Valley Police are preparing for 100,000 spectators to be in the town to try and catch a glimpse of Prince Harry and his bride, Meghan Markle.

But it will be nothing like the security operation for the wedding of Harry's brother Prince William and Kate Middleton in London in 2011.

The security operation for the wedding of the Duke and Duchess of Cambridge cost police more than PS6 million ($A11 million).

Nearly PS3 million was spent on overtime costs alone, with hundreds of officers drafted in to help police crowds watching the event.

A Thames Valley Police spokesman said costs for security for Prince Harry's wedding and any additional infrastructure would be shared between the force and the local council.

Security barriers are already being put in place and an elaborate police deployment has been mapped out.

It's fairly easy to establish a secure perimeter around the castle grounds, and a no-fly, no-drone zone will be enforced to keep the airspace clear.

But Harry and Meghan are determined to ride outside the castle in an open horse-drawn carriage, which calls for extra protective measures.

There are already police with automatic weapons visible on the streets of Windsor, a jarring fact of life reflecting that Britain's official terrorism threat level is set at "severe", indicating an attack is judged highly likely.

Before the spate of extremist attacks on Britain last year, it was common to see some royals in the streets and in the parkland surrounding the town.

Many of the military units Prince Harry has associations with will also be on the ground in Windsor on Saturday.