By Kevin Anderson
BBC News Online in Washington
Barbara Thomas travelled from Michigan, half way across the United States, to pay her respects to Ronald Reagan.
Secret service agents were out in force during the state funeral procession
Only hours before the funeral procession was to begin, she was sitting in front of the US Capitol waiting for her husband when suddenly police came running.
"They were shouting, 'run, run, run'," Mrs Thomas said.
She abandoned her lawn chairs and was rushed along with members of Congress, foreign dignitaries, other mourners and the media.
A small plane had violated the air exclusion zone around Washington. In the Capitol, police warned the crowd: "You have one minute to impact."
With a plane so close to the Capitol airspace, "we declared aircon red, and all the buildings are then dumped," said US Capitol Police Officer Michael Killough.
At the Supreme Court, several justices were rushed away in cars while workers were herded into the basement.
Media set up to cover the Reagan funeral procession were sent tripping over cables in the rush, as a pair of F-15 fighter jets on air patrol was called on to intercept the plane.
It took only minutes for authorities to determine the plane was carrying a dignitary, was cleared to land and had temporarily lost radio contact, but the area was cleared in what some were calling the most alarming evacuation since the 11 September attacks.
Mrs Thomas said: "Security is so tight. The police are on a hair trigger."
'Special security event'
The security surrounding President Ronald Reagan's funeral is being described as unprecedented.
Thursday - Lying in state continues all day and night
0700 Friday - Public viewing ends
1030 Friday - Departure from Capitol
1130 Friday - Funeral service at National Cathedral
1345 Friday - Motorcade leaves cathedral
1425 Friday - Departure from Andrews Air Force base
2115 Friday - Private burial in Simi Valley
All times US Eastern Daylight-Saving Time, four hours behind GMT
The Department of Homeland Security has declared the memorial events a "special security event".
"It is a sad commentary when the observation of a memorial service for a former president of the United States must be labelled a national security special event," Attorney General John Ashcroft told a Senate hearing.
"Such is the fact of modern life in Washington and such is the nature of the war against al-Qaeda," he added.
The declaration places the Secret Service in charge of 5,000 officers from a number of agencies, including the Washington Metropolitan Police Department, the US Park Police and the US Capitol Police.
The memorial events come only two weeks after Mr Ashcroft warned that al-Qaeda was planning to carry out attacks in the United States in the next several months.
The event presents an attractive target to terrorists and a daunting challenge to the thousands of security officers.
Some 20 heads of state were expected for the national funeral to be held on Friday, the largest number since the 50th anniversary of Nato in Washington in April 1999, according to Washington Police Chief Charles Ramsey.
Upwards of 200,000 people are expected to queue to pay their respects to the late president as he lies in state at the Capitol.
Security near the queue was tight. "We flooded the area with law enforcement where the line is," Capitol Police Officer Killough said.
In preparation for the national funeral on Friday, security officers swept the National Cathedral and road closures were announced.
At the state funeral on Wednesday, the security presence was obvious.
Secret Service agents with bomb-sniffing dogs inspected media vehicles and equipment.
Bomb sniffing dogs roamed the crowds and inspected media vehicles
Blackhawk military helicopters patrolled the skies, and snipers could be seen atop several buildings.
Mourners queuing up to view the casket in the Capitol rotunda were told not to bring cameras, bags or backpacks to simplify the security screening process.
In a subway station near the Capitol, a machine was sampling the air to detect the presence of chemical or biological agents.
The Washington police department also activated its Joint Operations Command Centre and its network of wireless CCTV cameras.
The network of 14 wireless cameras focuses on public areas on the National Mall, the US Capitol, the White House and major highways in downtown Washington.