Washington ? The body of former President Ronald Reagan will arrive here today for three days of elaborate state funeral ceremonies that present a daunting security challenge, as officials balance how to let an expected 150,000 people mourn his passing while preventing terrorist attacks on what must be a tempting target.

The Department of Homeland Security has given these events its highest security designation, on a level with presidential inaugurations and this summer’s national political conventions. Bomb-sniffing dogs will scour the National Mall. Military, federal and local police will patrol the area, which will include a 16-block funeral procession today through the streets of the nation’s capital.

Metal detectors have been set up outside the U.S. Capitol. Every person will have to pass through one before filing past Reagan’s flag-draped coffin as it lies in state under the Capitol Rotunda for approximately 36 hours. Reagan’s body then will be taken to Washington National Cathedral for a funeral ceremony Friday that will be attended by President Bush and numerous heads of state.

Police, soldiers and workers were busy this week preparing for Washington’s first state funeral since former President Lyndon Johnson’s in 1973 — a time long before the Sept. 11 terrorist attacks when mourners could file into the Capitol without being checked for weapons.

Security fencing was installed around the National Cathedral this week, and barricades sprang up along the parade route. Because the series of ceremonies is an official national security event, the U.S. Secret Service is in charge of protection from the arrival of Reagan’s body to its departure Friday.

Reagan’s iconic image and the large crowds coming to honor him could make the events an inviting terrorist target.

But al-Qaida typically spends years planning major attacks, and the suddenness of Reagan’s funeral and the increased security attending it would make it difficult for them to attack successfully, said James Carafano, a homeland security expert with the Heritage Foundation, a conservative think tank in Washington.

“As a target, it’s kind of difficult to take advantage of, because they don’t know what the security is going to be like, they don’t know what the arrangements are like,” he said. “An orchestrated terrorist attack is really unlikely.”