At least six people were reported killed and more than 300 people injured when a shallow 5.9-magnitude earthquake struck northwestern Iran early on Friday, according to state TV.
The United States Geological Survey (USGS) said the quake struck around 60km (35 miles) from the town of Hastrud, in East Azerbaijan Province, shortly before 2:30am local time on Friday (2300 GMT Thursday).
"Rescue teams and helicopters have been dispatched to the quake-hit areas and hospitals are on full alert to help injured people. Unfortunately six people were killed and 345 were injured," Iran's emergency medical services chief Pirhossein Kolivand told state TV.
The USGS issued an alert warning that "significant casualties are likely and the disaster is potentially widespread".
State news agency IRNA said the quake was followed by more than 60 aftershocks, causing panic among residents who rushed out of their houses in the middle of the night.
The quake was felt in several towns and cities in Iran. The European-Mediterranean Seismological Centre (EMSC) said the quake was felt by some 20 million people. The quake had a shallow depth of 10km (6.2 miles), which would have amplified the shaking.
"Many of people were injured while trying to flee their houses in panic," a local official told TV, adding that some 400 houses were damaged.
Images and videos posted online showed a collapsed structure in Mianeh, also in East Azerbaijan Province near the epicentre of the quake.
Residents were also seen camping outside their homes or in their cars.
Khabar Online also showed several livestock killed in Mianeh.
Press TV quoted Mohammad Baqer Honar, crisis management chief of the province, as saying that eight rescue teams were deployed to the areas hit by the quake.
Criss-crossed by several major fault lines, Iran is one of the most earthquake-prone countries in the world.
The country has suffered a number of major disasters in recent decades, including at the ancient city of Bam, which was decimated by a catastrophic earthquake in 2003 that killed at least 31,000 people.
In 1990, a 7.4-magnitude quake in northern Iran killed 40,000 people, injured 300,000 and left half a million homeless, reducing dozens of towns and nearly 2,000 villages to rubble.
Iran has experienced at least three other significant quakes in recent years - one in 2005 that killed more than 600 people and another in 2012 that left some 300 dead.
In 2017, at least 600 people were killed when a 7.3 earthquake hit Iran's Kermanshah province.