President Saddam Hussein of Iraq, besieged by tenacious domestic rebellions that continued to threaten his control of key cities, Saturday promised broad political reforms that he said would transform his totalitarian regime into a multiparty democracy. In his first national address since he announced his army's withdrawal from Kuwait Feb. 26, the Iraqi leader offered to create a new constitution, Cabinet and Parliament as well as a break from his Baath party's 23-year monopoly on power. "Our decision to build a democratic society based on the constitution, the rule of law and a multiparty system is a decisive, irrevocable decision," he said in the televised, hourlong speech. He mentioned no timetable for implementing what he called "a new stage" in the nation's development. The speech, in which Hussein asserted that Iraqi armed forces had smothered an uprising by Shiite Moslem rebels in the south and would soon snuff the revolt by Kurds in the north, apparently was an attempt to add a political component to the fierce military counterattack that Hussein has unleashed against the rebels. It also served as a response to an unprecedented gathering in Beirut this week of exiled Iraqi political leaders, who pledged to construct a multiparty democracy in the country if they gain power. Genuine democratic-style pluralism and parliamentary democracy is foreign to Iraq's modern political tradition and would be a radical departure from Hussein's brand of one-man leadership. In a reminder of how tenuous Hussein's position appears to be, Kurdish rebels Saturday claimed they now control Mosul, Iraq's third-largest city and a key military and industrial center. The report could not be independently verified. Although U.S. officials have acknowledged Kurdish guerrilla advances throughout northern Iraq, senior analysts in Washington Friday doubted Kurdish claims that they had won control of any Iraqi cities.