A 7.4-magnitude earthquake struck off the coast of Guatemala on Wednesday, leaving some 15 people trapped in rubble while 15 others were feared dead in the southwest of the country.

The earthquake rattled nerves in neighboring Mexico and El Salvador, sparking a tsunami alert on the Salvadoran coast and evacuations from offices, homes and schools as far north as Mexico City.

Guatemalan President Otto Perez said three people were confirmed dead but preliminary reports indicated that another 100 people were missing in San Marcos, a department 250 kilometers (155 miles) west of the Guatemalan capital.

"Other preliminary information, which has not been confirmed, says the number of dead could rise to around 15 people. But this is preliminary data and we have not confirmed it," he told a press conference.

Fire department spokesman Sergio Vazquez told AFP earlier that "around 10" people were found dead in San Pedro Sacatepequez, a town in San Marcos.

The region bore the brunt of the damage as cars were crushed, roads cut off and utility services collapsed.

Communication was difficult in the area due to downed powerlines and power outages, Vazquez said. Some 73,000 homes were without power, the energy minister said.

The US Geological Survey said the quake struck at 1635 GMT some 24 kilometers south of Champerico and 163 kilometers west-southwest of Guatemala City. The depth was 41.6 kilometers.

The Mexican Seismological Service said 35 aftershocks followed the quake, which it put at a magnitude of 7.3, some 68 kilometers southwest of Ciudad Hidalgo in the state of Chiapas.

The quake was strongly felt in Guatemala City and southern Mexico. People streamed out of homes, schools and office buildings in Mexico City too.

"We just had an earthquake," Mexico City Mayor Marcelo Ebrard wrote on Twitter.

"It was intense in a good part of the city," he said, adding later that the quake did not cause any damage in the sprawling metropolis of 20 million people.

Mexico City's metro service was briefly suspended.

Buildings were also evacuated in the southern Mexican states of Oaxaca and Chiapas, without reports of damage or victims.

"I was scared. It was horrible," said Uvita Mena, who lives in the Chiapas town of Tuxtla Gutierrez.

The USGS had initially measured the quake at magnitude 7.5.

The Pacific Tsunami Warning Center said there was no destructive widespread tsunami threat, but Salvadoran President Mauricio Funes ordered evacuations in Puerto de la Libertad and other western coastal towns.

"This country is constantly exposed to threats... and this time the threat is a tsunami on the Salvadoran coast, especially the western beaches," Funes said.

The Salvadoran Environment Observatory issued a tsunami alert for the west coast departments of Ahuachapan and Sonsonate.

"We are not talking about a tsunami of a large magnitude," observatory spokeswoman Daysi Lopez said, adding that it would be very localized.

The event came two months after a 7.6-magnitude earthquake rocked Costa Rica, without causing any casualties or injuries.