People observe the damage at a building in Mexico City after a strong quake hit Mexico.

The quake struck south of the Pacific resort of Acapulco, between the states of Guerrero and Oaxaca, and was followed by a nearby aftershock of 5.1, the US Geological Survey said. Initially it had estimated the magnitude at 7.9.

"There are no reports of serious damage by the quake," President Felipe Calderon wrote on his Twitter account.

Humberto Calvo, from Guerrero state civil protection services, said Acapulco was free of damage but he warned: "The problem could be in some areas between Guerrero and Oaxaca (states). We're checking."

Guerrero state governor Angel Aguirre told Milenio television two hours after the quake that more than 500 homes were damaged or destroyed in southern Mexico.

"We don't have human losses in the states of Guerrero and Oaxaca," Aguirre said, adding that authorities were checking the state of schools and public buildings near Ometepec, the town nearest the epicenter.

The White House said President Barack Obama's 13-year-old daughter Malia, who is on vacation in Mexico on spring break, was safe.

"In light of today's earthquake, we can confirm that Malia Obama is safe and was never in danger," said Kristina Schake, spokeswoman for First Lady Michelle Obama.

The Pacific Tsunami Warning Center said a destructive, widespread tsunami had not been generated but warned of possible "local tsunami effects."

In Mexico City, buildings swayed, telephone and power lines were cut off and traffic lights stopped working as office workers rushed onto the streets.

"I stood up when I saw the lights moving," said Ana Fernandez, an office worker in the central Roma district.

"Our boss told us to get out and we followed instructions not to shout, run or push. I was really scared but I made myself stay calm."

German tourist Gernot Nahrung said he was in the city's Chapultepec park and did not feel the long swaying movement of the quake, which lasted several minutes. "My mum told me" 'It's shaking, it's shaking,'" he said.

Mexico City Mayor Marcelo Ebrand said no serious damage was visible during a helicopter survey.

The mayor's Twitter account said the water system and other "strategic services" were not experiencing problems.

Some windows broke at the city's international airport and the monorail between the two terminals was stopped temporarily.

Local radio Formato 21 reported one person injured by the collapse of a pedestrian bridge on a bus which was not carrying passengers, in the north of the city.

It was one of the strongest quakes to shake Mexico City since 1985, when an 8.1 quake left between 6,000 and 30,000 dead, according officials and rescue organizations respectively.

"We were told to evacuate around 50 people," said office worker Francisco Bernal. "The earthquake was strong but now we're prepared, unlike in 1985," he added.

The quake's epicenter was inland at a depth of 12.4 miles (20 kilometers), 100 miles (162 kilometers) from the tourist city of Oaxaca, according to the USGS.