Sexual abused by Borders Patrol.

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Despite some improvements in the way complaints of abuse are handled, people detained by Federal immigration agents along the United States-Mexican border are still often subject to ''cruel, inhuman or degrading treatment,'' according to a lengthy report released today by the human-rights group Amnesty International.

The abuse includes beatings, sexual assault, racially derogatory comments and denial of medical care and food, said the group, which conducted several weeks of research along the border, including interviews with immigrant advocacy groups, immigration officials and people who said they were victims of brutality.

Accusations of abusive behavior by its agents have long dogged the Border Patrol, whose responsibilities include detaining Undocumented Immigrants and, in many cases, deporting them back to Mexico or other countries from which they came. The report today suggested that many immigrants were unable or afraid to report abuse by agents, and that the relatively few cases in which agents have been arrested or disciplined for such behavior represent only the tip of a much larger problem.

The report was issued exactly one year after the shooting death of an American teen-ager in Redford, Tex., by a member of a Marine patrol backing up Federal agents in an anti-drug operation. Two grand juries have declined to indict the marine who fired the fatal shot, but many groups that promote immigrants' rights say the killing was an unjustified homicide that should result in criminal charges. And the report today said that a ''thorough, independent investigation'' of the incident was still needed

''Although instances of civil and human rights violations by I.N.S. employees are not common,'' she said in a statement, ''any instance of abuse is one too many and will not be tolerated.'' She also said the agency was already carrying out several steps recommended by an official advisory panel that includes private citizens and a representative of the Mexican Government.

The report, ''Human Rights Concerns in the Border Region with Mexico,'' was the first major look at the issue by the group, which has chronicled human-rights abuses around the world. It said that all detainees should be informed of their rights, in their native languages, and that they should not be ''discouraged, threatened or prevented from exercising their right to file a complaint.''

At a news conference the Texas border city of McAllen, representatives of Amnesty International and several other human-rights groups issued lists of dozens of cases in which immigrants had complained of abuse. These included statements by people who said they were struck with batons, fists or feet, often as punishment for trying to run away from Border Patrol agents; denied water, food and blankets for those detained at Border Patrol stations, and sexually abused or threatened with sexual abuse. The reports of ill treatment came from men, women and children, virtually all of them of Latin-American descent.

We are here to send a strong message,'' said Kerry McGrath, an Amnesty International official. ''We want these officials to know that the world is watching.''

The group said that complaint forms were not readily available and that ''there is a perception that I.N.S. officers act with impunity.'' And it said that while the Border Patrol had sharply increased the number of agents along the border in recent years, it had not had a corresponding increase in the number of personnel investigating reports of abuse.