Thousands of peace demonstrators are expected to mark the 40th anniversary of the bombing of Hiroshima by encircling Washington's Mall and monument areas tomorrow, enclosing the Ellipse, the Capitol and the Pentagon with a 15-mile-long strip of cloth.

However, impact on traffic in the areas involved is expected to be minimal, according to D.C. police and organizers of the demonstration, known as "The Ribbon."

Plans call for the ribbon to be formed some time after 2 p.m. tomorrow by tying together about 15,500 individual cloth panels carried by marchers in the demonstration. The anniversary of the first use of a nuclear weapon in war is Tuesday.

The panels, each a yard wide, were made by groups and individuals throughout the United States and in many foreign countries, as personal protests against atomic war.

The individual strips were intended as their creators' depiction of what they could not bear to see lost in such a war, the organizers said. In addition to their symbolic and often vividly emotional messages, most bear a nine-inch strip of cloth tape sewn to the corners to facilitate tying.

"The ribbon will only be tied together in places where it will not obstruct traffic," said Marie Grosso, an organizer. Roads crossing the perimeter formed by the ribbon will be open, she said.

The demonstration was conceived three years ago by Justine Merritt, a 61-year-old Colorado woman. Demonstration-related activities, including folk singing and an appearance by Hiroshima survivors, will begin at 10 a.m. at the three assembly sites for marchers -- the Capitol, the Lincoln Memorial and the Pentagon.