After an initial impasse, high ranking Interior Department officials and leaders of the "Longest Walk" Indian movement have reached in unusual agreement that will permit up to 3,000 Indians to use the washington Monument grounds and other parklands for eight days of political demonstrations and religious ceremonies in July.

Spurred by Sen. James G. Abourek (D.S.D.), the two sides hammered out a rough outline of the agreement at a meeting in Abourek's office late Monday.

Organizers of the Longest Walk who have been coordinating the trek of hundreds of Indians across the country since last winter, say they are coming to Washington to demonstrate their culture to non-Indian America and to protest what they say is anti-Indian legislation currently before Congress.

Prior to the agreement reached Monday, Indian leaders and local National Park Service officials had met twice about demonstration arrangements, but the officials balked at the Indians' demand that they be permitted to "camp" in the city. The Indians had threatened to come anyway.

Park Service officials called a third meeting Monday, but the Indians representative did not show up.

Abourezk, who is chairman of the Senate Select Committee on Indian Affairs and had been monitoring the situation, then ralled in Interior Deputy Assistant Secretary David Hales, National Park Service Director William J. Whalen, other officials and Indian leaders late Monday to break the impasse.

The agreement reached calls for the Indians to make a ceremonial march into the city on July 15, conduct a continuous four-day religious ceremony and vigil from July 16 to 19 in West Potomac Park near the Lincoln Memorial and participate in outdoor workshops and conferences on the Washington Monument grounds from July 20 to 22.

Most of the 3,000 expected will camp at Greenbelt Park in suburban Maryland near Interstate 95 and the Capital Beltway, about 12 miles from downtown Washington. They will come each day to the monument grounds and West Potomac Park, then return at night to Greenbelt. Abourezk said he has asked the Pentagon to provide military buses for transportation but has received no commitment as of yesterday.

For the religious ceremonies in West Potomac Park, the Indians plan to erect 50 tepees, two council lodges and an altar with a continuously burning symbolic fire. The ceremonies will be attended by about 200 elders and other Indian officials.

Clyde Bellecourt, a cofounder of the American Indian Movement and national coordinator of the Longest Walk, said rallies, marches and demonstrations also are planned against a series of bills in Congress that are designed to abrogate U.S. treaties with the Indians and deny them various fishing, grazing and mineral rights.

Bellecourt and other organizers stressed what they said is the peaceful and spiritual nature of the Longest Walk and said it lacks the confrontational quality of the 1972 Trail of Broken Treaties protest here that resulted in a four-day takeover by the Indians of the Bureau of Indian Affairs.