A 12-year-old Mexican-American boy, his hands cuffed behind him, was shot in the head by a Dallas policeman five years ago, and color photos of the dead youth left President Carter visibly shaken as he met with Hispanics seeking greater protection from police brutality in the Southwest.

"This is something that any American would be ashamed of," the president was quoted as saying Friday night after a brief pause and a bow of the head upon seeing the two side, view photos of Santos Rodriguez seated dead in the front seat of a police car.

The president was shown the pictures by state Rep. Ben T. Reyes in a 45-minute meeting between Carter and 23 Mexican-American leaders here during the president's overnight stay for a Democratic fund-raiser.

The president informed them he had reached Attorney General Griffin Bell by telephone Friday afternoon to direct Bell to tak e personal charge of the U.S. Justice Department's civil rights probe into the death of Rodriguez. Rodriguez was killed by a policeman playing Russian roulette with a .357 magnum revolver after the officer had detained the youth during an investigation on July 24, 1973.

A Dallas newspaper reported Friday that the Justice Department had decided not to prosecute the officer, Darrell Cain, on civil rights charges. Cain is serving a five-year murder sentence on a state court conviction, and the statute of limitations for federal prosecution runs out on July 24.

Carter assured the group Friday, however, that the Dallas newspaper account was incorrect and that no decision has been made.

On other matters, Carter committed himself to more federal executive appointments and more federal judge ships for Americans of Mexican ancestry, according to the Hispanic leaders. "You will have Mexican-American judgeship - I am committed to you," the president was quoted as saying.

Carter, the leaders said, listened cordially to their complaints on other matters - the administration's plan to control illegal immigration and governmental reorganization of immigration responsibilities - but committed himself to no changes in them.

Mexican-Americans are the predominant minority in five southwestern states, where they have historically been abused and discriminated against.

They voted 97 percent, for Carter in 1976 and by the mid-1980s, Hispanics nationwide are expected to be the country's largest minority group.