Hundreds more are settling in for a night on the streets of Windsor to secure the best viewing spot.
The couple will wed at noon at St George's Chapel in Windsor Castle, before travelling through the town in a carriage at about 13:00 BST.
In her vows, Ms Markle will not promise to "obey" her husband and Prince Harry has chosen to have a wedding ring.
Prince Charles will walk Ms Markle down the aisle, after her father, Thomas, was unable to attend for health reasons.
On the eve of their wedding, Prince Harry told crowds in Windsor he was feeling "relaxed" and Ms Markle said she was feeling "wonderful".