The mysterious posters appeared overnight last week, pasted on walls and utility boxes all over Washington, and many people found them objectionable: They said, "Meese is a Pig."

But if you missed the dozens, or even hundreds of signs directed at Attorney General Edwin Meese III, don't bother searching the neighborhood. Most of them have been torn away or defaced by spray paint in the last two days.

And nobody seems to know who put them up.

Justice Department spokesman Patrick Korten called the poster, which has foot-high red lettering, "obnoxious" and "not nice," but added, "What can you do about it? It's a free country."

Employes with the D.C. Department of Public Works began tearing off the posters affixed to city property this week after a complaint from "Capitol Hill," a department official said.

The signs appeared to be mostly in the Northwest section, with many of them on upper Connecticut and Wisconsin avenues, around Dupont Circle and Adams-Morgan, and throughout downtown. Others were spotted in Montgomery County.

The posters contained a commentary in smaller type on the bottom, which upbraided Meese's personal ethics and approach to law enforcement, noted his relationships to people connected to the Wedtech scandal, and called him a "weasel."

Korten and D.C. officials, as well as representatives of Metro and local police departments, were puzzled about who put up the signs in an apparently organized fashion during nighttime strikes last week. Meese has attracted protest from a range of groups since joining the Reagan administration almost seven years ago.

Also this week, Meese has been the target of a harsh denunciation in newspapers published by the organization of political extremist Lyndon H. LaRouche Jr.

The group blames Meese for the Justice Department's prosecution of its leader and several top aides on charges of obstruction of justice. The LaRouche trial started in Boston last week.

A spokesman for the LaRouche group could not be reached for comment.

In the recent articles, written by LaRouche, Meese is described as a Soviet dupe who is "weak and confused" and commits "atrocities." LaRouche says that Meese's prosecution of LaRouche "stinks to high heaven," and, in a refrain repeated many times in the articles, that the U.S. practices "Mickey Meese justice."

Staff writer Rene Sanchez contributed to this report.