A powerful 7.8-magnitude earthquake in Ecuador has killed at least 77 people and injured more than 500, Vice-President Jorge Glas says.

The quake struck at 18:58 local time on Saturday (23:58 GMT) near the northern coastal town of Muisne.

Widespread severe damage is reported, with a bridge destroyed as far south as Guayaquil about 300km (190 miles) away.

A state of emergency has been declared in six provinces and the National Guard has been mobilised.

The quake, the country's largest in decades, also shook buildings in the capital, Quito.

President Rafael Correa is cutting short his trip to Italy to return home. He called for calm, saying on Twitter: "Our infinite love to the families of the dead."

Mr Glas said that at least 77 people had died and 588 had been injured, adding that the figures could rise as a number of affected areas had not yet been reached.

He called for calm, particularly in the city of Portoviejo, amid reports of a "lack of public order".

Gabriel Alcivar, mayor of the town of Pedernale, which is close to the epicentre, said: "We're trying to do the most we can but there's almost nothing we can do."

He said dozens of buildings had been flattened and looting had broken out.

"This wasn't just a house that collapsed, it was an entire town," Mr Alcivar said.

Carla Peralto, a resident of Boyaca, one of the worst-affected areas, told the BBC: "I never felt something like that in my life. It was so strong. I was feeling very, very scared... I was thinking 'God, please stop that because maybe I die today'."

Serious damage was also reported in the city of Manta, with an airport tower among the buildings destroyed.

Manta resident Ramon Solorzano told Reuters: "Most people are out in the streets with backpacks on, heading for higher ground. The streets are cracked. The power is out and phones are down."

The US Geological Survey said the earthquake struck at a fairly shallow depth of 19.2km (11.9 miles), about 27km from Muisne in a sparsely populated area.

There have been a number of aftershocks, the biggest at 5.4.

The Pacific Tsunami Warning Centre has now issued a message saying that the threat of a tsunami has now mostly passed and that any remaining risk should be evaluated by local authorities.

However, it earlier said tsunami waves reaching 0.3 to one metre above the tide level were possible for some coasts of Ecuador. Any damage that had occurred might not be visible until daylight.

Quito resident Zoila Villena told the Associated Press news agency: "I'm in a state of panic. My building moved a lot and things fell to the floor. Lots of neighbours were screaming and kids crying," she said.

Parts of the capital were left without electricity for some time.

Reports say a big oil refinery has been temporarily shut as a precautionary measure.

Neighbouring Peru had 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 precautionary measure.

Ecuador is prone to earthquakes. It is located along the so-called Ring of Fire, a horseshoe-shaped, seismically turbulent area of the Pacific Ocean.