A strong 7.4 magnitude quake struck off the Pacific coast of Guatemala, El Salvador and Mexico on Wednesday, prompting panicked evacuations, but there were no immediate reports of casualties or damage.

The US Geological Survey (USGS) said the quake struck at 1635 GMT some 24 kilometers (15 miles) southwest of Champerico, Guatemala and 163 kilometers (101 miles) west-southwest of Guatemala City.

The quake was strongly felt in Guatemala City and Chiapas, and prompted evacuations as far north as Mexico City, where thousands of people streamed out of buildings.

"We just had an earthquake," Mexico City Mayor Marcelo Ebrard wrote on Twitter. "It was intense in a good part of the city."

One Mexico City metro line was briefly suspended but Ebrard reported moments later that the system was back running normally.

The Pacific Tsunami Warning Center said there was no destructive widespread tsunami threat.

The USGS had initially measured the quake at 7.5 magnitude.