A "robust" security strategy for the Royal Wedding is likely to include snipers on roofs, police divers in the sewers and a crackdown on known trouble-makers.
Security expert Peter Bleksley told TV ONE's Breakfast the uniformed police managing the one million people lining London's streets will be just a small part of the overall operation.
"There have been numerous units checking out the Thames, there have been divers checking out the drains and the sewer covers have been welded shut, there have been helicopters doing reconnaissance trips, every inch of London will be checked and double checked," he said.
World leaders, celebrities and dignitaries are on the guest-list for the ceremony in Westminster Abbey and Britain is currently at its second highest threat level of "severe", meaning an attack by militants is considered highly likely.
Around 5,000 police officers will be on duty to deal with potential threats ranging from international Islamist militants to anarchists and stalkers.
Senior police officers said they would not tolerate anyone who attempted to disrupt the event, which will be viewed by as many as 2 billion people on television worldwide.
"We intend to interact robustly, quickly and firmly if anybody engages in any criminal activity," London Police's Assistant Commissioner Lynne Owens said.
Senior officers have spoken of their fears that anarchists, who took part in riots in the capital last month, might also target the occasion, although groups contacted by Reuters have indicated that they would not bother.
Last December, a car carrying William's father Prince Charles and his wife Camilla was attacked during violent protests by students. More than 60 people arrested in recent demonstrations have been banned from London on the day.
Bleksley said known trouble-makers will be watched closely on the big day.
"If you've been taking part in recent demonstrations and disruptive events happening in London, you're a known extremist, possibly an anarchist, then it is highly likely on Friday morning as you come out your door there will be a surveillance team outside your house," he said.
Specialist royal protection teams and armed officers will be on duty as part of overt and covert plans, and Bleksley believes army personnel could also be working undercover to "have numbers on the ground".
Banners with slogans deemed to be offensive will be confiscated and people in London will be stopped and searched. Even guests going to the wedding will need to pass through a security check before they can take their seats.
"Last year we had the Pope visit the UK so all the systems that will be put in place on Friday have had a dress rehearsal," Bleksley said.
But the security expert sounded a note of caution about the operation.
"All the resources of the State have been put in to making this day a happy day, and the Metropolitan police were very bullish that they would keep it safe," he said.
"I would like to be happy and confident but the Met police have some large failings on their CV, I hope Friday isn't one."
Last week, police said they had banned the radical group Muslims Against Crusades (MAC), whose members include some of Britain's best-known Muslim extremists, from holding a protest outside Westminster Abbey where the ceremony is to be held.
Another unnamed organisation has also asked to hold a protest.