The storm ripped up trees and power pylons, tossed vehicles and left swathes of debris across the northern region of Halkidiki as the severe weather brought gale-force winds, heavy rain, and hail.
"It is the first time in my 25-year career that I have lived through something like this," Athansios Kaltsas, director of the Nea Moudania Medical Center, told a Greek television station. "It was so abrupt and so sudden."
Two of those killed were Russian natives, two were Romanian and another two were from the Czech Republic, according to government spokesman Stelios Petsas.
A state of emergency was declared in the region, a three-finger peninsula near the northern city of Thessaloniki popular with tourists in the summer.
The two Russians -- a man and a boy -- died after a tree fell near their hotel in the town of Potidea, while a woman and an eight-year-old boy from Romania were killed after a roof collapsed on a restaurant in Nea Plagia, Sky News reported. Another two were killed by falling trees.
Around 140 people were injured in the storms, with 65 treated by paramedics. Petsas said 23 people remained hospitalized Thursday, including one woman in critical condition in intensive care.
Crews were working Thursday to restore power that was knocked out to 80 percent of the area slammed by the storms, while the fire department said it had received more than 600 requests for assistance, including for rescues, to cut fallen trees and pump water from flooded basements.
The coast guard said patrol boats were searching for a 62-year-old fisherman missing off Halkidiki's coast since Wednesday night after he took his fishing boat out in the afternoon before the storm struck.
Prime Minister Kyriakos Mitsotakis, who was elected on Sunday and just formed his government, appointed Public Order Minister Michalis Chrisochoidis to head the response effort, Petsas said, while the army was assisting repair crews
The region had seen temperatures in the upper-90's in the days before the storms.
Meteorologist Klearxos Marousakis told Sky News the conditions were "extremely unusual" for this time of year, as Greece usually experiences hot and dry summers.
The Associated Press contributed to this report.