Syrian President Bashar al-Assad tried desperately Sunday to find a new grip on the country that has been slipping away from him the past several weeks, dispatching troops to the sites of clashes between police and opposition demonstrators and promising through surrogates to repeal a controversial law that allowed Syrians to be held without trial. Assad, who has remained mostly out of sight throughout the weeks of unrest, during which more than 60 people have been killed by his regime, was expected to address the country Sunday night. (An adviser said “within 24 to 48 hours,” but there has been no sign of an announcement.) Meanwhile, state television stoked sectarian discord by calling the uprising a plot by the United States to take Syria back to the Stone Age. At least six people were killed in clashes with the government Sunday in the coastal town of Latakia, and more than 100 were injured. A heavy military presence in Damascus, the nation’s capital, gave the city a nervous feel, and the government sent text messages warning citizens to stay away from Umaweyeen Square.