GUATEMALA CITY — The first tropical storm of the 2010 season hit the Pacific coastline of Guatemala and Mexico on Saturday, killing 12 people under landslides and rockfall triggered by torrential rains.

Tropical Storm Agatha’s rains caused a landslide in a precarious hillside settlement of Guatemala City that killed four people and left 11 missing, Guatemalan disaster relief spokesman David de Leon said. Most of the city was without electricity at nightfall, complicating search efforts.

Four children were killed by another mudslide in the town of Santa Catarina Pinula, about six miles outside the Guatemalan capital.

And in the department of Quetzaltenango, 125 miles  west of Guatemala City, a boulder loosened by rains crushed a house, killing two children and two adults, de Leon said.

Agatha formed early Saturday in the East Pacific and moved over land in the evening along the Guatemala-Mexico border, said the U.S. National Hurricane Center in Miami.

The center of the storm was 20 miles east of Tapachula on Saturday night, moving northeast at 10 mph and packing winds of 40 mph.

More than 850 people were evacuated from their homes because of flooding affecting much of central and southern Guatemala.

Before the rains, Guatemala already was contending with heavy eruptions from its Pacaya volcano that have blanketed the capital in ash and destroyed 800 homes. Officials expressed concerns that Agatha’s rains could exacerbate the damages.

Though Tropical Storm Agatha was expected to lose force as it came ashore overnight Saturday, it still could bring rains of 10 to 20 inches and as much as 30 inches in isolated areas of Guatemala.

“The storm will start to weaken, and we hope that on Sunday it will be just a tropical depression,” said Romero Garcia of Guatemala’s Meteorological Institute. “That is not to say that there won’t be heavy rains.”