(Adds details, quotes, updates strength and location)
By Mike McDonald
GUATEMALA CITY, Nov 7 (Reuters) - A strong earthquake off the coast of Guatemala on Wednesday shook buildings in Guatemala City and San Salvador and was felt as far away as Mexico City, Reuters witnesses said.
Firefighters said eight people were injured when a building collapsed in San Marcos, 101 miles (163 km) west-southwest of the capital, but there were no immediate reports of fatalities.
Landslides were blocking roads in some areas, they added.
The Pacific Tsunami Warning Center said there was no widespread tsunami threat from the quake, which occurred at 10:35 a.m. (1635 GMT).
The magnitude 7.4 quake struck off the Central American nation's Pacific coast, 15 miles (24 km) south of Champerico, Guatemala, near the border with Mexico, the U.S. Geological Survey said.
The epicenter was 26 miles (42 km) below the surface, according to the USGS. It initially reported the quake as magnitude 7.5.
A Reuters witness in Guatemala City said people were evacuating residences in parts of the capital, and firefighters and rescue workers were on alert.
"It was really big, I felt quite nauseous," said secretary Vanessa Castillo, 32, who was evacuated from her 10th-floor office in Guatemala City.
Hundreds of people were on the streets making phone calls to friends and family.
Office workers were also hurrying out of buildings in Mexico City. Mayor Marcelo Ebrard said the quake was felt strongly in a large part of the city of 20 million people.
"There are evacuations in various parts of the city," he said via Twitter. (With reporting by Nelson Renteria in San Salvador; Writing by Krista Hughes; Editing by Simon Gardner and Xavier Briand)
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