The left-wing activist made the announcement in Jordan, where some 20 prominent volunteer Arab lawyers are based to defend Saddam.
Clark held office in the 1960s under President Lyndon Johnson, and has since become an outspoken critic of U.S. foreign policy on Iraq. He visited Saddam in Baghdad in February 2003 just before the U.S.-led invasion.
He denounced the special tribunal established to try members of the former regime as a creation of the U.S. military occupation that had no authority in law as a criminal court.
More recently, he has offered legal advice to numerous figures, including former Yugoslav president Slobodan Milosevic.