About 300 demonstrators, walking behind a large painting of a black South African carrying a dead child, denounced South African apartheid in a march and rally in South-Central Los Angeles. Organized by the Coalition to Commemorate the Sharpeville Massacre, a bloody confrontation 25 years ago when South African police fired upon and killed 69 blacks, the demonstrators included blacks, whites, Asians and Latinos. Nineteen blacks died from police gunfire in South Africa last week during a march there to commemorate the same event. "We hope this demonstration will be a strong and clear signal to the Reagan Administration that his policy of constructive engagement (toward South Africa) is not working, that it is a hoax," said coalition spokesman Tony Hall. Among the scores of placards carried by the marchers were several demanding that the South African Consulate in Beverly Hills be closed.