A small tsunami struck the Seferisar district of Izmir, said Haluk Ozener, director of the Istanbul-based Kandilli Observatory and Earthquake Research Institute.
Six people were killed in Izmir, including one who drowned, and 202 were injured, according to Turkey’s Disaster and Emergency Management Presidency.
Izmir governor Yavuz Selim Kosger said at least 70 people had been rescued from the wreckage. He said more than 10 buildings had collapsed, while others were damaged.
Search and rescue efforts are continuing in at least 12 buildings, rescuers said.
Four light injuries were reported on Samos.
Turkey’s health minister Fahrettin Koca said 38 ambulances, two helicopters and 35 medical rescue teams were working in Izmir.
The European-Mediterranean Seismological Centre said the quake had a preliminary magnitude of 6.9, with an epicentre 13 kilometres north-north-east of Samos.
The United States Geological Survey put the magnitude at 7.0. It is common for preliminary magnitudes to differ in the early hours and days after a quake. Multiple aftershocks struck the region.
Izmir mayor Tunc Soyer told CNN Turk that about 20 buildings had collapsed. The city is the third biggest in Turkey with about 4.5 million residents.
Environment and urban planning minister Murat Kurum said people were trapped under the wreckage and rescue efforts were under way.
Videos posted on Twitter showed flooding in the immediate aftermath of the quake in the Seferhisar district. Turkish officials and broadcasters called on people to stay off the streets after reports of traffic congestion.
Turkish media showed the wreckage of a multiple-storey building in central Izmir, with people climbing it to start rescue efforts.
Victims were pictured being carried from the rubble of collapsed buildings, and smoke was filmed in several spots in central Izmir.
Turkish media said the earthquake was felt across the regions of Aegean and Marmara, including Istanbul. Istanbul’s governor said there were no reports of damage in the city, Turkey’s largest.
The quake was felt across the eastern Greek islands and as far as the capital Athens, and in Bulgaria.
Greek seismologist Efthymios Lekkas told Greek state television ERT that it was still too early to say whether this was the main earthquake, although he said it was likely.
“It is an event that is evolving,” mr Lekkas said, adding that some damage had been reported in parts of Samos.
A tsunami warning was issued, with residents of the Samos area told to stay away from the coast. Water rose above the dock in the main harbour of Samos and flooded the street.
Residents have been told to stay away from buildings as aftershocks rattled the area. Local officials told state media there were reports of damage to buildings and part of the island’s road network.