A force of more than 6,000 to guard the royal wedding

The main threats include the possibility of a terrorist outrage by Irish republican extremists, threatened disruption and disorder by anarchists and an extremist group called Muslims Against Crusades.

British police officers inspect tubes to be used to build a stage reserved for media outside the Westminster Abbey in central London.

LONDON // One of the biggest security operations in British history is being rolled out for Friday's wedding between Prince William and Kate Middleton.

Almost 5,000 police officers and 1,350 troops, plus rooftop snipers, undercover agents, helicopters, patrol boats and sniffer dogs, will protect the royal couple and their guests.

"It is the most comprehensive security operation I have ever known," a senior police source told The National. "And it has to be because we are facing such a diversity of potential threats.

"It is not just that the couple themselves who might be targets. There are about 80 heads of state and foreign royals attending, any one of whom could potentially be at risk from dissident groups in the UK with their own grievances."

Those wedding guest dignitaries include Sheikh Mohammed bin Zayed al Nahyan, Crown Prince of Abu Dhabi, and a close friend of Prince Andrew, the bridegroom's uncle, since the pair were pupils at Gordonstoun School in Scotland.

Scotland Yard's concern, though, stems from the possibility that UK-based groups of foreign dissidents will protest the presence of visiting dignitaries from their homelands, although the withdrawal from the guest list last Sunday of the crown prince of Bahrain has quelled fears of unrest from the "Arab spring" spilling on to the streets of London.

However, the main threats are home grown and include the possibility of a terrorist attack by Irish republican extremists, threatened disruption and disorder by anarchists and an extremist group called Muslims Against Crusades.

Police admitted yesterday that the Muslim group had refused to agree to conditions on a protest march during the wedding and had also failed to attend a meeting with police last week.

A banner on the group's website warns Prince William and his younger brother Harry, who are both members of Britain's armed forces, to "watch their backs".

But it's an attack by a mentally disturbed individual that worries the security service most, said the police source speaking on terms of anonymity.

"This is the person who does it because he - or, occasionally, she - is obsessed by the royals in some way or, alternatively, just wants a moment of what they see as 'glory'.

There is a specialist unit that tracks these sort of people. At least, they track the ones we know about. What worries security services throughout the world are the ones we do not know about.

"And, of course, the difficulty is that this is a great public occasion. You have to balance the overriding right of the public to see the royal couple and celebrate on the streets, with the need to keep everyone safe. It is not easy."

The processional route between Buckingham Palace and Westminster Abbey, where the wedding is taking place at 11am BST (2pm UAE), will be lined with about 1,100 soldiers and 900 uniformed police.

Other officers and soldiers will be in the procession but most of the police will be in the background in rapid response squads that can head to any area where there is trouble, or patrolling the streets along and around the procession route.

The latter groups will use random stop-and-searches, closed-circuit television cameras and "pre-emptive policing" to quash any perceived security threats.

Powers included in pre-emptive policing allow officers to detain anyone on terror charges if they believe he or she poses a threat, even if the officers do not have hard evidence.

These sort of measures would never be acceptable in the United States, former CIA agent Mike Baker told Fox News last week.

"There's more mature acceptance in the UK of the trade-off between civil liberties and security," he said. "They went through home-grown terror issues with the IRA and they were bombed in World War Two, and it developed an attitude that you don't find here (in the US)."

Email and telecommunications traffic intercepted by the security services has indicated considerable "chatter" over plans to disrupt the wedding, though officers believe that a great deal of this is little more than braggadocio.

However, the threat to turn the wedding day into "a nightmare" by Muslims Against Crusades (MAC) - a small, extremist group which caused outrage by burning symbolic poppies on the day last November when Britain honoured its war dead - is being taken seriously by police.

The group had a request turned down by police to hold a protest outside Westminster Abbey at the time of the wedding but has now threatened to hold a protest elsewhere along the route. This, in turn, has prompted the far right, Islamophobic group, the English Defence League, to threaten to take counter measures.

Mainstream Muslim groups in Britain have expressed revulsion at the MAC plans. "We believe their action on this national occasion of celebration is completely at odds with the ethos of Islam. We remind them of Prophet Muhammed's blessed words, that 'marriage is indeed half of faith'," the Muslim Council of Britain said in a statement.

"They have chosen a day when the whole nation will be watching in celebration and cause offence and stir friction amongst the Muslim and non-Muslim communities. We as a community need to stand united against such extremist groups and convey to the nation that this is not Islam."

Similarly, anarchists operating under a loose-knit umbrella group called UK Uncut, whose members rioted and caused considerable damage to several West End shops last month during an otherwise peaceful march by several hundred thousand trade unionists through London, are also threatening mayhem.

Under the slogan, "Reclaim the Royal Wedding", they are urging anti-monarchists to demonstrate outside Westminster Abbey during the service.

And although the security services are understood not to have intelligence of any specific threat to the day's events by Irish terror groups, police fear that the event could be a tempting target for dissident republicans who have recently stepped up a campaign of bombings and murder in Northern Ireland.

Police have already checked and sealed possible hiding places for bombs along the route, including drains, lampposts, traffic lights and rubbish bins.

Commander Bob Broadhurst, one of the police officers in charge of security for the wedding, told a parliamentary committee earlier this month that Scotland Yard was watching out for every threat "from terrorism downwards".

He described the main danger as the possibility of an attack on "the principals" - police jargon for the royals and other VIPs attending the wedding.

"The threat to the wedding is a threat to principals...it is a threat to democracy," he added.