CARACAS, Venezuela (Reuters) - Thousands of supporters of Venezuelan President Hugo Chavez marched in Caracas on Saturday to support the leftist leader in his dispute with Mexico's president over U.S. free trade proposals. State workers, unionists and students, many wearing red T-shirts, waved flags and anti-U.S. placards as they marched through the capital accompanied by trucks blaring revolutionary songs, Venezuelan folk ballads and Mexican mariachi music. Venezuela and Mexico withdrew their ambassadors on Monday after Chavez called his Mexican counterpart, Vicente Fox, a "lap dog" of U.S. imperialism for his close ties to Washington and told him, "Don't mess with...