Barak Obama may be a people's president, but for his inauguration, he will be separated from the public by a security bubble that most experts believe is impenetrable and virtually fool-proof.

The induction of the nation's first Afro-American president, coming in the first White House transition since the September 11 attacks and just weeks after the terrorist strike in Mumbai, poses special concerns, United States law enforcement and security officials say.

The US secret service has probably done everything possible to safeguard not only the president-elect and his entourage, but the parade route, the National Mall and other locations that will be part of Obama's short but symbolic path to the presidency. For instance, Obama will be riding in a new limousine nicknamed 'The Beast', considered the most secure ever, and virtually impervious to chemical or biological attacks and rocket-propelled grenades.

And as with all major events, the secret service has spent months working with dozens of local, state and federal agencies on security, crowd control and logistical support. Thousands of extra police officers and military troops are being brought in from around the country, and measures to protect against chemical and biological attack will be in place, along with decontamination tents.

At least 150 multi-agency "intel teams" will be deployed throughout the region so undercover FBI agents and behaviour specialists can look for trouble. Of particular interest would be individuals or small groups of men with backpacks lurking in large crowds, or entering the Metro from distant suburban stations, like those who launched deadly attacks in London's subways in 2005.

In some places, Washington, DC, will look like an occupied city. Sharpshooters will be on virtually every building. Law enforcement and intelligence nerve centres and mobile command posts are sprouting. The FBI is deploying a scary-looking armoured assault vehicle and a weapons-of-mass-destruction response truck.

The military, supporting civilian authorities, is using sophisticated new surveillance systems developed for the Iraq and Afghanistan wars to monitor the mall. A P-3 Orion reconnaissance plane will fly above the mall to collect information that instantaneously can be transmitted to teams on the ground in the event of a threat. The system is known as ''Rover''.

The Pentagon also would station cameras and other detectors on buildings around the mall on inauguration day to ensure a constant picture, defence  officials said. They added that they would be able to do much more than watch the crowd. They can sense radiation associated with a dirty bomb and in some cases even detect a conventional explosive device.

In addition to response, officials have worked on prevention. Authorities have been scouring the internet and other gathering places for extremists to look for signs of trouble brewing. So far, they say, they aren't seeing any.

But they still dread the 'X-factor' - intangibles that they cannot control and that could upend their most carefully laid plans by panicking the immense crowd.

At the top of that list, they say, is the lone wolf individual or small group who could slip through the intelligence and security net. A burst of gunfire or an explosion could cause significant casualties or pandemonium. But the chance of that is considered extremely unlikely. Law enforcement and intelligence officials say they have seen nothing to suggest the president-elect is being targeted.

Yet, as one senior Federal Bureau of Investigation official said, ''We can prepare and we can prepare, but there are always variables. It only takes one person to come in and cause havoc.'' Many disgruntled individuals fly under the radar, they say, keeping violent thoughts and plans to themselves and out of Internet chat rooms that have been monitored for years by undercover agents. They could easily hide among the millions of visitors expected to overload transit systems and wedge into every nook and cranny of indoor and outdoor space.

Domestically, white supremacists have discussed Obama in potentially threatening terms since early in his candidacy. Threats increased after he won the primary and again after he won the November election. Obama has attracted attention not only because of his race, but due to what extremists believe to be his ties to or sympathy for Islam, Israel and even his support for gun control measures.

But in recent weeks, the threatening 'chatter' has died down, according to law enforcement officials and the Southern Poverty Law Centre, which is active in tracking US-based extremist activity. Current and former security specialists say, however, that such screening procedures usually can't catch the kind of zealots that Obama might attract.