A powerful earthquake shook El Salvador Tuesday, toppling hundreds of buildings and killing at least 174 people in a country still mourning more than 800 who died in an even stronger quake exactly one month ago. The quake flattened much of the heart of San Vicente, a city of 40,000 about 35 miles east of San Salvador. Its streets were buried under mountains of debris when adobe homes collapsed in the quake. Nearly all of the deaths occurred in the hardest hit area - the adjoining provinces of San Vicente, Cuscatlan and La Paz, in the centre of the country east of San Salvador, officials from the National Emergency Committee are quoted by the Associated Press as saying. That area was less seriously damaged in January's quake. The city of San Vicente is the capital of the province of the same name. The earthquake struck at 8:25 a.m. and was centred halfway between San Vicente and San Salvador. The U.S. Geological Survey measured its magnitude at 6.6, a strong but not as powerful as the 7.6 magnitude temblor that killed at least 844 people last month. ''It is true that this is another blow for El Salvador, but I call for tranquility. We have to be calm,'' President Francisco Flores said in an interview with The Associated Press during a helicopter tour to assess the damage.