Mexican authorities said at least three people were killed when a strong earthquake hit the southwestern coastal state of Guerrero on Saturday night and shook buildings in the capital.

The temblor, measuring 6.8 on the Richter scale, occurred at 19:47 local time (0147 GMT Sunday), the National Seismological Service of Mexico said.

The epicenter of the quake was located about 42 km southwest of Iguala in Guerrero and about 166 km south-southwest of Mexico City, with a depth of 64.9 km, according to the U.S. Geological Survey (USGS), which had estimated the magnitude at 6.8 before downgrading it to 6.7 and then 6.5.

At least three people were killed in the quake, which sent residents rushing into the streets and interrupted public traffic and subways in Guerrero.

According to Guerrero's Civil Protection agency, an 11-year-old boy was killed when a house's roof collapsed, a second died in the small town of Ixcateopan and a driver was killed on the high way when a rock rolled down and hit his car.

When the quake happened, panicked residents ran out of buildings and gathered in the streets. Some people began to cry and pray. The quake also interrupted public traffic and subway in Guerrero.

Helicopters have been dispatched to check the situation, officials said.

Marcelo Ebrard, mayor of Mexico City, said by Twitter that "there is power cut in part of the city but no major damage has been reported."

Power supplies have not been restored in 30 regions of Mexico City, where some 70,000 residents are affected.

A monstrous 8.1-magnitude earthquake hit the country in 1985 and killed as many as 10,000 people and wrecked parts of Mexico City.