At the funeral in suburban Rio Piedras, family members said they hoped the case's notoriety would boost the international campaign against capital punishment.
"God chose my uncle to change history," said Jackeline Nieves. "Now the death penalty isn't seen as something normal. It's seen as the worst, most inhumane method."
Family and friends wore two colored ribbons : one black, to symbolize their mourning, and the other orange, in tribute to the prison uniform Diaz wore during more than two decades on death row.
Medical experts said the 55-year-old convict could have experienced severe pain as needles that were supposed to inject drugs into his veins were instead pushed all the way through the blood vessels into surrounding soft tissue.