About 400 supporters of Immaculata Preparatory School urged last night that Archbishop James A. Hickey block the planned closing of the 79-year-old Catholic, all-girls school.

If the archbishop doesn't act soon, their leaders said at a meeting in the school gymnasium, the group will take legal steps to stop the sale to American University for $7.6 million.

"American University took the deal and we're going to take it back," said Charles Wolf, an Immaculata parent who heads the fund-raising committee for a new fund set up to save the school. The protestors did not spell out what legal action they might take.

Archbishop Hickey has been out of town for 10 days, but before leaving he appointed a task force of six church officials to discuss the planned sale with parents. Hickey is expected to return this weekend.

The 8.2 acre campus, occupied by Immaculata Prep, a high school, and Immaculata Dunblane, a grammar and junior high school, is at Tenley Circle, Wisconsin and Nebraska avenues NW. The schools were established at the site in 1905 and together enroll about 560 girls. They are scheduled to close in June 1986 with settlement and transfer of the property to follow.

The schools are owned by the Sisters of Providence, an order of Catholic nuns who said the sale was necessary to provide funds for the care of their aging members.

The sale was first publicly disclosed on Oct. 2, but leaders of the Sisters of Providence said they had signed a binding letter of intent to sell late last year. Although the order is self-supporting and separate from the Archdiocese of Washington, Sister Ann Doherty, superior general of the order, said the sale had been approved in June by Archbishop Hickey, head of the Washington archdiocese and shortly afterward by the Vatican.

Last night, James Spelman, president of the Immaculata Fathers Club and head of the new Save Immaculata -- Dunblane Fund, said he had received word from lawyers for the sisters that a final contract of sale with American University had been signed on Monday.

On Wednesday, Spelman said leaders of the fund had a long distance conference call with the governing council of the order, which is based in St. Mary of the Woods, Ind., urging them to sell the school instead to the parent group. Spelman said none of the nuns told them about the final contract.

"That was a listening session," an official of the order said last night. She said there was no agreement to hold a dialogue on the sale.

Arthur Schissel, another parent, said the group was urging Hickey to bring together AU Richard Berendzen and Sister Ann Doherty to cancel the sale. If they don't, he said Hickey should withdraw his approval.

"It's possible that the Archbishop could use his moral suasion on our behalf for a change," Schissel said.

" . . . Right now that would be a big leap forward for the archbishop," he said to applause.