The letter seeks a permanent ban on cloning to create babies and a temporary moratorium on cloning to make embryos for medical research.
"A moratorium would allow time for alternatives to research cloning to be investigated," the letter stated.
Among those who signed the letter were Andrew Imparato, president of the American Association of People with Disabilities. He said unregulated cloning could "foster a market for designer babies where disabled children are undervalued."
Another signer, Judy Norsigian, co-author of the women's sexual health classic "Our Bodies, Ourselves," said cloning could encourage poor women to take fertility drugs so they could sell their eggs to researchers.
Proponents of cloning for medical research said a moratorium would prevent medical breakthroughs.
"A moratorium or ban on therapeutic cloning would slam the door on promising research that could ease the suffering of millions," said paralyzed actor Christopher Reeve in a news release responding to the letter.
The letter was distributed by the Oakland-based Center for Genetics and Society, which lobbies on biotechnology issue.