Outgoing President Michelle Bachelet said that she had reports of five deaths so far and could not rule out that there might be more.
The US issued an initial tsunami warning for Chile, Peru and Ecuador. That was later extended to Colombia, Antarctica, Panama and Costa Rica.
Japan's meteorological agency has warned of a potential tsunami across large areas of the Pacific.
President Bachelet called on people to remain calm and contact the authorities if they needed help.
Four deaths have been reported in Maule and one in Bio Bio. Ms Bachelet, who has now gone into an emergency meeting, said that there were areas of the country where communications were down and teams were working to restore them.
Buildings in Santiago were reported to have shaken for between 10 and 30 seconds, with the loss of electricity and communications.
The US Geological Survey (USGS) said the earthquake struck at a depth of about 35km. It said tsunami effects had been observed at Valparaiso, west of Santiago, with a wave height of 1.29m above normal sea level.
One journalist speaking to Chilean national television from the city of Temuco, 600km south of Santiago, said many people there had left their homes, determined to spend the rest of the night outside. Some people on the streets were in tears.
Mark Winstanley, who contacted the BBC from Vina del Mar, 100km north-west of Santiago, said buildings had shaken and electricity and phone connections were cut but he could see no structural damage yet.
A university professor in Santiago, Cristian Bonacic, said that this was a massive quake but that the cities seemed to have resisted well. Internet communications were working but not mobile phones.
Chile suffered the biggest earthquake of the 20th century when a 9.5 magnitude quake struck the city of Valdivia in 1960, killing 1,655 people.
Original article posted here.
Update 1: The US government said a tsunami warning had been issued for Chile, Peru and Ecuador. (source)
Update 2: The earthquake has been upgraded to magnitude 8.8. (source)
Update 3: The Pacific Tsunami Warning Center issued a tsunami warning for Chile and Peru. The center recorded a tsunami wave as high as 9 feet. (source)
Update 4: Six confirmed dead (source).
Update 5: President Bachelet has declared a state of catastrophe, 47 confirmed dead (source).
Update 6: Death toll reaches 52, more casualties expected (source).
Update 7: Casualties reach 64 (source), EU offers assistance to victims (source).
Update 8: 78 reported dead (source).
Update 9: If you're concerned about family or friends in Chile, you can contact the State Department at 1-888-407-4747
Update 10: Casualties are now up to 85 in the Maule region alone (source).