(Updates death toll, adds collapsed houses, stranded tourists)
By John McPhaul
SAN JOSE, Costa Rica, Jan 8 (Reuters) - A strong 6.1-magnitude earthquake struck Costa Rica on Thursday, killing a teenager as well as two children selling candy near a national park, stranding hundreds of tourists and damaging buildings in the capital.
The quake triggered landslides in rural areas and tore apart a highway near the Poas national volcano park. Some 300 tourists were seeking shelter for the night in a valley where they had been visiting a waterfall when the road out was destroyed.
Two young sisters selling candies at the volcano were buried in a landslide and died, said Jorge Jimenez, a spokesman for the Red Cross. A teenage girl died when her home on the side of the volcano was engulfed by a landslide.
Several other people were reported to have been hurt in villages northwest of the capital, San Jose.
"I was very frightened. First I got underneath an arch support and then, when it calmed down, I got out of the house," lawyer Michael Henreichs, 35, said in San Jose.
National Emergency Commission spokeswoman Rebeca Madrigal said there were fears some people in villages near the volcano park could be trapped in houses that collapsed during the quake.
The quake's epicenter was 20 miles (32 km) from San Jose at a depth of 21.7 miles (35 km), the U.S. Geological Survey said, and caused shaking for around 40 seconds.
Pieces of ceiling fell off homes in San Jose and television images showed buildings with shattered windows and damaged walls, but there were no reports of widespread injuries.
Earthquakes above magnitude 6 can cause widespread damage in populated areas.
Initially measured at 6.2, Thursday's tremor cut off power to parts of San Jose and some businesses evacuated employees. Images from a TV studio showed ceiling lights swaying.
The National Coffee Institute said it had no reports of major damage to the country's coffee farms.
Costa Rica is a popular tourist destination due to its lush natural parks, volcanoes and rich wildlife, but is prone like the rest of Central America to natural disasters.
A group of six British volcanology students with three Costa Rican guides were inside the crater of the Poas volcano when the quake struck, but all were unharmed, Jimenez said.
Aerial television pictures of Vara Blanca, where the tourists were stranded, showed groups of people standing around bonfires in the valley, waving for help, but officials said there was no way of reaching them until the morning.
Dan Whitlock, an American doing missionary work in Costa Rica, said the earthquake was so strong even in the capital that guests at his hotel stumbled as they ran out.
"I was outside and all of a sudden I could see the whole building shaking," he said. "You could see the pool water moving like a tsunami." (Additional reporting by Robert Campbell and Noel Randewich; Editing by Peter Cooney)