A time bomb blast caused extensive damage at an American military store in the center of Athens, police said. There were no injuries in the blast outside the PX, or post exchange, where American military personnel buy their goods, they added. Witnesses said the blast smashed windows in the hangar-sized building, uprooted trees on the sidewalk and shattered nearby storefronts. An extreme leftist Greek guerrilla group called Revolutionary Popular Struggle claimed responsibility for the explosion in a telephone call to an Athens newspaper, according to police. The group has been responsible for a number of recent bombings in the Greek capital.