A military walk-through of Friday's royal wedding route and a music rehearsal is taking place.
Members of the armed forces taking part in the parade were due to turn out at 0430 BST in full ceremonial uniform at Wellington Barracks in Westminster.
Carriages were expected to take part in the procession to Westminster Abbey, but the band will not play.
A music rehearsal at the abbey, where Prince William and Kate Middleton are getting married, was due to follow.
The abbey closed its doors to the public on Tuesday so that preparations could get under way.
The route of the carriage procession on 29 April will pass along The Mall, Horse Guards Road, Horse Guards Parade, through Horse Guards Arch, Whitehall, along the south side of Parliament Square and into Broad Sanctuary.
The Metropolitan Police Service said there would be "extensive" road closures from about 0200 BST to 0700 BST during the rehearsal.
A dress rehearsal for clergy and broadcasters is also scheduled for Wednesday, while the royal couple are expected to hold their own preparations with senior clergy at the abbey.
Miss Middleton was pictured driving from her parents' house in Bucklebury, Berkshire, to London on Tuesday.
Meanwhile, the first member of the public has begun camping out in front of Westminster Abbey.
John Loughrey, 56, began his week-long vigil on Monday night, saying he planned to stay in his position to ensure a prime spot for the event.
As well as the Royal Family, 50 heads of state are attending the ceremony, which it is anticipated will be watched by up to two billion people on television. There will be 70-80 close protection teams for VIPs on the day.
On Tuesday, detectives leading the major royal wedding security operation appealed for the public to be the "eyes and ears" of the 5,000 officers tasked with maintaining law and order on the big day.
Scotland Yard's appeal came alongside a warning that anyone attempting to disrupt the wedding in central London on Friday would face a "robust" response.
The police are continuing to negotiate with radical Islamist group Muslims against Crusades over proposed protests, after its application for an event at Westminster Abbey was rejected.
Nationalist group the English Defence League had threatened to hold a counter-demonstration if the police granted permission to Muslims against Crusades. It has since said that no counter-protest will be held.
Elsewhere, a separate protest group with Middle Eastern links has warned police it is planning disruptions.
Police have powers to ban any major protests along the main route that the royal couple will take but are unable to rule out "static" protests taking place at other nearby locations.