* Quake one of biggest to hit Mexico since 1985 disaster
* Scores of buildings damaged, five injuries reported
* President says no reports of serious damage (Updates number of buildings damaged)
By Cyntia Barrera and Dave Graham
MEXICO CITY, March 20 (Reuters) - A major earthquake struck Mexico on Tuesday, unleashing panic as it damaged hundreds of buildings and caused homes in the capital to bounce like "trampolines".
Office workers fled into the street when the 7.4-magnitude quake shook Mexico City for more than a minute. Cell phone lines went down, building were evacuated, traffic snarled and the stock exchange had to suspend trading early.
The governor of the southwestern state of Guerrero, Angel Aguirre, said he had received reports of 500 homes damaged, with some of them knocked down, but he gave no more details.
The tremor was one of the strongest since the devastating 8.1-magnitude earthquake of 1985, which killed thousands in Mexico City.
No deaths were reported on Tuesday and the quake caused no major disruptions to air travel or to oil installations, but it scared many residents.
Martha Suarez, an Argentine living in the capital's Roma neighborhood said she had never known anything like it.
"My TV set fell over, the building felt like it was on a trampoline. This one was like no other I have felt before," Suarez said, holding her little dog close.
Emergency services said 800 houses were damaged in Guerrero state, many of them in Ometepec, near the epicenter of the quake. Officials in Guerrero, which is home of popular Pacific beach resort Acapulco, could not say if buildings had collapsed.
In the neighboring state of Oaxaca, 68 mud-brick houses were damaged and five people were injured, one of them seriously, in the hardest hit area around the town of Pinotepa Nacional near the Pacific coast, local emergency services said.
Some buildings in the capital's trendy district of Condesa were cracked by the earthquake on Tuesday, and residents raced out of buildings with young children and dogs in their arms.
"I swear I never felt one so strong, I thought the building was going to collapse," said Sebastian Herrera, 42, a businessman from a Mexico City neighborhood hit hard in 1985.
Television images showed part of a bridge collapsed onto a vehicle on the outskirts of Mexico City, but Mayor Marcelo Ebrard said no one was injured and that helicopter flyovers showed there was no sign of major damage in the capital.
President Felipe Calderon also said there were no reports of serious damage.
More than two hours after the quake, some residents of the capital were still too scared to return to their homes, fearing powerful aftershocks.
Mexico City's international airport was operating normally and only a couple of flights to the United States were temporarily grounded, a spokesman said.
State oil company Pemex said all its installations on the Pacific coast were operating normally, including the country's largest 330,000-barrel-per-day capacity Salina Cruz refinery.
The U.S. Geological Survey, or USGS, said the quake registered magnitude 7.4 and there were several aftershocks.
In Acapulco, schools were evacuated and some parents rushed to pick up their children, but there appeared to be no major damage to hotels or other buildings in the resort.
The White House, which has declined to confirm reports that President Barack Obama's daughter Malia was vacationing in Mexico, said the 13-year-old was safe.
"In light of today's earthquake, we can confirm that Malia Obama is safe and was never in danger," said Kristina Schake, communications director to first lady Michelle Obama. "We would reiterate our request that the media respect the privacy and security of the Obama children and not report on or photograph the girls when they are not with their parents."
The quake was felt as far away as Guatemala City.
A spokesman for Mexico City's health authorities said hospitals and clinics were operating normally although some patients were evacuated from damaged buildings.
The Pacific Tsunami Warning Center said the inland earthquake would not generate a destructive widespread tsunami, but there was the possibility of some local tsunami effects. (Additional reporting by Patrick Rucker and Mica Rosenberg in Mexico City, and Luis Enrique Martinez in Acapulco; Editing by Anthony Boadle and Kieran Murray)