The National Park Service is moving against demonstrators who camp out in front of the White House with a new rule that prohibits leaving parcels for more than an hour and only allows the use of hand-held protest signs.

The rule, which went into effect yesterday and carries a penalty of six months in jail and/or a $500 fine, is aimed at demonstrators who frequently maintain a constant vigil with bags and equipment in front of the White House. Specifically, they no longer will be allowed to place signs against the White House fence or anywhere else on the sidewalk there. They also can't leave bags or packages unattended for more than one hour.

National Park Service spokesman Sandra Alley said the new standards are interim rules and will not be permanent until May 23, the deadline for public comments about them. A notice of the rule printed in yesterday's Federal Register said such signs "represent potential threats to the security of the area," the president and the White House, because they "can potentially conceal explosives."

An Interior Department employe said the new rule was prompted in part by the Dec. 8 incident in which nuclear arms protester Norman Mayer was killed by authorities after a siege in which he threatened to blow up the Washington Monument with explosives he never had.

The notice in the Federal Register said the incident "highlighted the threat that could be potentially caused to the White House and the President by one determined individual."

The register notice also noted that a pile of equipment "impedes the free flow of pedestrian traffic."

Another reason for the rule, Alley said, is "the aesthetics of the White House, so that people can see the White House. We don't think this violates any rights. They are still free to express whatever message they want to express."

A handful of antinuclear demonstrators in front of the White House yesterday, replete with a few of Mayer's old protest signs, was outraged by the announcement. The legal director of the local American Civil Liberties Union office said he will look "closely" at the new rule.

"They may have legitimate security concerns," said Art Spitzer, the ACLU legal director, but added "I think when they pretend to be concerned about the free flow of traffic, that's crap."

"It's unconstitutional," said Concepcion Picciotto, a native of Spain and a current D.C. resident protesting nuclear arms. "If you want to have a 24-hour vigil you can't hold the signs all that time."

Alley, the National Park Service spokesman, said protesters will be given a grace period of a few hours to move their signs and parcels for the next few days.