By Kevin Anderson
BBC News website, Washington
Washington has mounted an unprecedented security operation for the first presidential inauguration since the 11 September 2001 attacks.
Military aircraft, boats and some 7,000 troops will provide security
Anti-aircraft batteries are protecting the capital from airborne threats.
Combat air patrols enforce a large no-fly zone. Coast Guard boats patrol the Potomac River.
Some 6,000 police, 7,000 military personnel and an undisclosed number of
Secret Service agents are on duty.
They are using the latest in hi-tech monitoring, surveillance and communications equipment as President Bush is sworn in for a second term on Thursday.
Outgoing Secretary of Homeland Security Tom Ridge said authorities knew of no specific threats against the inauguration or the accompanying balls, dinners and concerts.
But he added: "Security will be the highest levels it has ever been for any inauguration. We will have 24-7 surveillance of key inaugural facilities."
Authorities had reported that terrorist chatter had died down recently, but as inauguration week began, a report revealed fears that al-Qaeda would use limousines filled with gas canisters as bombs.
The limousine threat was revealed in a 38-page planning document seized by US intelligence from an al-Qaeda operative captured in the UK last year, Time magazine reported.
The gas canisters would be painted yellow to fool rescue and fire crews into thinking that toxic gas was involved and to maximise terror and chaos.
The inauguration is not mentioned in the plans, but US authorities reviewed the document in early January.
Limousine firms have been notified and barriers have been set up to prevent any "vehicle-borne improvised explosive devices".
Secret Service operation
The Secret Service is overseeing security, helping coordinate the security efforts of some 50 federal, state and local agencies.
Surveillance includes a new hi-tech command centre that takes in feeds from fixed video cameras and those mounted on helicopters all along the parade route from White House to the US Capitol.
Authorities are prepared for a range of potential threats including suicide bombers, a fire at the Capitol or a chemical or biological attack.
In addition to the hi-tech surveillance equipment, jamming equipment is being used that would prevent or delay the use of remote control to set off an explosive device.
Teams with bomb-sniffing dogs are combing the area, while special teams trained to rescue people from collapsed buildings,
nuclear experts and environmental clean-up crews are on standby.
Downtown shut down
Washington has become accustomed to high levels of security.
But the inauguration marks an entirely new level of disruption.
Security will be high, even for a city accustomed to terror alerts
The no-fly zone around Washington has been tripled in size: Usually, no private flights are allowed within 16 miles (26km) of the Washington Monument.
But on inauguration day private aircraft are banned in a zone 23 miles (37km) from Washington's three major airports, Reagan National, Dulles and Baltimore-Washington.
Commercial flights will still be allowed during the day, but military radar from ground stations and surveillance aircraft monitor the area.
Violators will be intercepted by combat aircraft or customs planes.
Authorities believe the greatest threat comes from a truck loaded with explosives.
For that reason, on inauguration day, some 100 blocks of downtown Washington have been closed to traffic, while with an additional 100 blocks have some form of restrictions imposed.
The closings began on Sunday to allow for a dress rehearsal of the inaugural parade, and small-scale closings continued throughout the week to allow for the president and foreign dignitaries to be ferried to and from concerts and balls.
The parade route was shut beginning the night before the inauguration so that manhole covers could be welded shut.
Biological and chemical air monitors have been spread throughout Washington - in more subway stations than usual, around the National Mall, and in buildings and streets near the parade route and Capitol.
This network of sensors will allow authorities to plot the path of plume from a chemical or biological weapon and move people to safety.
The entire security effort is expected to cost some $15m to $17m.