Warning that ''advances in biomedical technology must never come at the expense of human conscience,'' President Bush today delivered his strongest condemnation of human cloning experiments in a speech designed to persuade a sharply divided Senate to ban the research.

''As we seek to improve human life, we must always preserve human dignity,'' the president told an audience of nearly 200 lawmakers, religious leaders, bioethicists, scientists and patients gathered in the White House. ''And therefore, we must prevent human cloning by stopping it before it starts.''

It was Mr. Bush's second major address in eight months on morality in science. In August, he announced that the government would finance research on stem cells derived from human embryos, but limited the studies to cells that had already been established in laboratories. It was a careful compromise that angered many social conservatives, who oppose any stem cell research and who make up an important part of Mr. Bush's political base.

Now that cloning is before the Senate, the president's advisers say, Mr. Bush feels compelled to speak out, not only to reassure conservatives but also because he is deeply troubled by the science. Unlike embryonic stem cell studies, which can be conducted on embryos left over from infertility treatments, cloning for research involves creating and destroying embryos for the express purpose of experiments.