WAHINGTON, March 21--More than 1000 students at predominantly Negro Howard University continued their sit-in today, occupying the administration building for the third day in a row. University officials closed the school yesterday and are seeking a federal injunction to remove the protesting students as tresspassers.

Other students are fortifying their positions in school dormitories, vowing to violate a University order that they 'clear out" by midnight Friday.

At an afternoon press conference today, protest leaders listed four demands that they said must be met before the protest will end. They demanded that:

* Howard president James M. Nabrit Jr. resign,

* Afro-American history and culture be emphasized in the curriculum and that the University be closely linked with the community,

* Charges of misconduct during the March 1 centennial celebration against 39 students be dropped and that no disciplinary action be taken against students involved in the present protest,

* A judiciary system be established.

The protestors' three-man steering committee -- including Ewart Brown, president of the student assembly--presented the demands to administration representatives in a three-hour meeting started this afternoon. The demands were not accepted and a second meeting started at 7 p.m., which had not broken up by midnight.

Faculty members are acting as liasons between students and the administration. President Nabrit, a Negro, has not been seen on campus for the past three days and is in Puerto Rico, although no one here could confirm that.

Because of Nabrit's absence, steering committee members refused to attend the evening meeting with the administration. Brown said, "If the administration won't send their first team, we won't send ours." Other students attended in their place.

At a late afternoon closed meeting of the protestors, Brown said that if the injunction is issued, "we will be up against the full might of the federal government." The assembled students were somber and for the most part, silent.

The Student Bar Association of Howard's law school is also seeking an injunction: this one against the university officials to compel them to reopen the school.

One university official said that the administration does not really want the injunction, but is merely using it as a threat against the students. "The situation is simply too volatile."

Stokely Carmichael last night addressed the group inside the administration building. He said, "This protest is not just a matter of students and academic freedom. It is a case of Black students at a Black school fighting against an anti-Black education."

An estimated 3000 of Howard's students have vowed to stay in their dormitories, and some have built barricades around their buildings. Students in some buildings are taking turns keeping watch throughout the day and night. No city or university policeman have been sighted on campus.

More than 50 local Negro-owned restaurants and stores have contributed food and cooking utensils to feed the students staying in side their dormitories.