Detectives leading the major royal wedding security operation have 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 to the hundreds of thousands of people expected in central London to celebrate Prince William and Kate Middleton's nuptials came as officers revealed another protest group with Middle Eastern links has warned it plans disruptions at the royal wedding.
The Metropolitan Police is already negotiating with extremist campaign groups Muslims Against Crusades (MAC) and the English Defence League over proposed protests.
Preparations for the big day continue and Westminster Abbey, the venue for the wedding ceremony, closed its doors to the public today so work can begin in earnest.
A full music rehearsal will take place at the Abbey tomorrow, a dress rehearsal for clergy and broadcasters on Thursday and at some stage the prince and his fiancee will hold their own preparations with senior clergy at the abbey.
Assistant Commissioner Lynne Owens, who is leading the major security operation, made the appeal to the public.
"We want them to be our eyes and ears," she said, adding: "If you see anything or anyone in the crowd that is acting suspiciously, please bring it to the earliest attention of our officers... they are there to help you.
"We are very clear that we want this to be a safe, secure and happy event, and we intend to act robustly, quickly and firmly if anyone engages in any criminal activity."
All the groups applying to protest have been told by officers that they are likely to be refused permission unless they agree to postpone demonstrations until later in the day.
MAC is understood to have refused the request and failed to turn up at a scheduled meeting with police chiefs last week, Ms Owens said.
As a result, MAC does not have an authorised demonstration on the day, the officer added.
A banner on the MAC website warns princes William and Harry to "watch their backs".
A string of famous sporting guests have been invited to the wedding but Ireland rugby captain Brian O'Driscoll has been forced to give up his seat to prepare with his Leinster team mates for a Heineken Cup semi-final match.
He initially said he would be joining wife Amy Huberman at the event but said later his presence depended on his team's progress in the competition, and they will now play French giants Toulouse in Dublin on Saturday.
The Dean of Westminster, one of the clerics who will officiate at the wedding service, said William's bride-to-be will have a "sense of calmness" when she arrives to marry her prince,
The Very Reverend Dr John Hall will be the first person to greet Miss Middleton after she emerges from her car following her journey to the venue.
He said he expected the couple to be feeling "a little bit nervous" but he would tell Kate as she prepared to walk up the aisle that "everyone was with her".
The Dean added: "I feel that she will have a sense of calmness. I'm sure there will be a great sense of excitement.
"It's important that she's enjoys it. It's important that Prince William enjoys it."