|Mexico quake captured on office camera|
The Pacific Tsunami Warning Centre said waves of 3.3ft (1m) had been measured off the port of Salina Cruz.
The quake has killed at least five people, including two children in the state of Tabasco.
Its governor, Arturo Nunez, said one was a baby who died at a children's hospital which lost power, causing the infant's ventilator to stop working.
The quake struck off Mexico's southern coast
Another three people were killed in neighbouring Chiapas state - among them two women who died in San Cristobal when a house and a wall collapsed.
"Homes, schools and hospitals have been damaged," state governor Manuel Velasco said.
People in the capital, Mexico City, ran out into the street in their pyjamas after the tremor shook buildings.
Power was extinguished in several neighbourhoods and windows were broken at the airport.
Buildings have been damaged at at least three people have been killed
|The quake struck off Mexico's southern coast|
It poses a potential tsunami threat to the Pacific coastlines of countries including Guatemala, El Salvador, Costa Rica, Nicaragua, Panama and Honduras, the US Tsunami Warning System said.
Buildings in Guatemala were shaken by the tremor.
Firefighters check through debris following the Mexico earthquake
"I had never been anywhere where the earth moved so much," said 31-year-old architect Luis Carlos Briceno, who is visiting Mexico City.
"At first I laughed, but when the lights went out I didn't know what to do. I nearly fell over."
It is the strongest earthquake to hit Mexico since a devastating tremor in 1985 that toppled buildings and killed thousands, the country's civil protection agency said.