At least 77 people have been confirmed dead so far, with another 578 injured, Vice President Jorge Glas wrote on Twitter.
Authorities say the powerful quake has caused "considerable damage" near the epicentre, as well as in the country's biggest city of Guayaquil, and knocked out power in much of the capital Quito.
A state of emergency had been declared nationwide.
A tsunami alert was issued but later lifted.
Hydroelectric dams and oil pipelines in the OPEC-member nation have been shut down as a precautionary measure.
In Pedernales, a town of 40,000 near the epicentre, dozens of buildings have been flattened, people are trapped and looting has broken out.
Mayor Gabriel Alcivar, who has pleaded for help, said: "This wasn't just a house that collapsed, it was an entire town."
Passengers at Guayaquil airport ran out of terminals when they felt the tremor.
"Lights fell down from the ceiling. People were running around in shock," said Luis Quimis, 30, who was waiting to catch a flight to Quito.
Maria Torres, 60, in northern Quito, said: "Oh, my God, it was the biggest and strongest earthquake I have felt in my whole life. It lasted a long time, and I was feeling dizzy. I couldn't walk ... I wanted to run out into the street, but I couldn't."
Among those killed was the driver of a car crushed by an overpass in Guayaquil.
President Rafael Correa, who is on a visit to the Vatican, called on Ecuadoreans to show strength while authorities monitor events.
The quake is the strongest to hit Ecuador in decades.
It struck at around 8pm local time on Saturday about 173km west-northwest of Quito and just 28km south-southeast of Muisne, the US Geological Survey said.
The quake also rattled northern Peru, according to authorities.
Across the Pacific in Japan, a 7.3 magnitude tremor struck Kumamoto province on Saturday, killing at least 32 people and causing widespread damage, in the second major quake to hit the island of Kyushu in just over 24 hours.