Vice President Jorge Glas said the quake hit at 6.58 p.m. on Saturday (11.58 GMT) near the northern coastal town of Muisne.
When he earlier put the death toll at 41, Glas warned that the toll would "unfortunately rise in the coming hours".
BBC reported widespread severe damage, with a bridge destroyed as far south as Guayaquil, about 300 km away from the capital Quito.
A state of emergency was declared in six provinces.
The earthquake, the country's worst in decades, shook buildings in Quito. There were a number of aftershocks, the biggest at 5.4.
President Rafael Correa cut short his trip to Italy to return home.
BBC quoted Glas as saying that at least 77 people had been killed and 588 injured.
The quake was centred just off the country's northwest coast at a shallow depth of 19 km, Xinhua news agency quoted the the US Geological Survey (USGS) as saying.
The USGS first put the quake at a magnitude of 7.4 and then raised it to 7.8. Ecuador's Institute of Geophysics put the magnitude at 6.5 and the depth at 10 km.
In Quito, the quake was felt for about 40 seconds, forcing people to flee to the streets in panic.
In the biggest port city of Guayaquil, a bridge collapsed on top of a car and the roof of a supermarket buckled, Xinhua reported.
In the coastal city of Manta, the airport was closed after the control tower suffered severe damages.
Residents in Manta said that many streets had cracked, power lines had snapped and telephone connections were down.
Reports say a big oil refinery had been temporarily shut as a precaution, BBC said.
Neighbouring Peru also issued a tsunami alert for its northern coastline.
The quake was felt in Colombia, where patients in a clinic in the city of Cali were evacuated from the building as a precaution.
Ecuador is located in a region with frequent volcanic eruptions and earthquakes.(IANS)