NEW YORK, April 26--An estimated 300 Columbia University students, entrenched for the fourth day in five classroom and administration buildings, won support today from the neighboring Harlem community and from black militants H. Rap Brown and Stokely Carmichael.

Brown, his face drawn and nearly milky pale, and Carmichael, in a blue pin-striped suit, led a crowd of 200 junior high and high school youths on a charge from Amsterdam Avenue, through police lines, and onto the besieged campus.

Police immediately sent for reinforcements and sealed off the campus, allowing only Barnard and Columbia students to enter through the outside gates. Other police guarded entrances 10 classroom buildings. Over 100 policemen were stationed around the area, and in nearby Harlem extra patrols roamed the streets.

On to Hamilton Hall

The group marched to Hamilton Hall, which has been held by 50 black students and an unknown number of community militants for the past three days. Brown later delivered a speech on the steps of the building as admirers cheered him on.

He said that he backed the demands of the demonstrators and threatened violence from the community if the demands are not met.

"We are going to let Columbia know that if they don't deal with the black brothers here, they're going to have to deal with the black people of Harlem," Brown said.

Mark Rudd, chairman of Columbia's chapter of Students for a Democratic Society, said at a press conference at 7:30 p.m. tonight that he would not settle for anything less than a complete amnesty for all participants in the demonstration. He also rejected a proposal for a tripartite commission of students, faculty, and administration to help resolve the conflict.

The demand for amnesty has now become the focus of the demonstration. One of the other three demands--stopping construction on a gymnasium in Morningside Park--has been met at least in part. The other two--ending ties with the Institute for Defense Analyses and dropping punishments for an earlier demonstration--are less important and are close to resolution now.

Vow to Stay

The 300 students inside the five buildings--Hamilton, Low Library, Avery Hall, Fayerweather Hall, and the mathematics building--again vowed to stay inside until their demands are met.

Although no official statement has yet been made on the progress of negotiations between the students and the administration, several reliable sources here indicate that Columbia offered to let all the demonstrators off with mere warnings. The students, however, insist on no punishment at all.

The Columbia Spectator, the university's daily newspaper, reported yesterday that sources indicated that the work stoppage on the gymnasium, announced yesterday as "temporary," will be permanent.

Today was the first day that members of the community bordering on Columbia actively participated in the demonstration.

In all, 400 teenage Negro youths assembled on the grassy quadrangle in the center of the Columbia campus. They chanted anti-war slogans, listened to the radio, and cheered on speakers.