Two of the victims were elderly Czech tourists who were killed when strong winds and water destroyed their camper van, according to police.
"For five minutes it was hellish", said Haris Lazaridis, owner of a tavern where a 54-year-old woman from Romania and her son were killed when the roof caved in. While a Russian Federation man & his 2-year old son died when a tree fell on their vehicle in Potidea. A Reuters correspondent saw sun loungers strewn in mangled heaps with other debris across Nea Plagia's beaches.
Prime Minister Kyriakos Mitsotakis, whose government was sworn in this week after winning elections on July 7, cancelled his morning schedule and was being continuously briefed on the situation, a government official said.
Wednesday night's storm in the northern Halkidiki peninsula ripped up trees and power pylons, overturned vehicles and left swathes of debris across the coast.
Weather of this kind is not the norm in Greece at this time of year, when it is typically hot and dry.
"We mourn for the loss of these souls", said Citizen Protection Minister Michalis Chrisochoidis, who is overseeing operations in the area.
Athansios Kaltsas, the director of the Nea Moudania Medical Centre, told Greek television that it was "the first time in my 25-year career that I have lived through something like this".
A picture taken on July 11, 2019 shows broken trees fallen on cars after a storm in Nea Plagia, in Chalkidiki, Northern Greece.
He said his patients ranged from 8 months in age to 70 years old, and some had suffered head injuries from falling objects.
Powerful storms raged over Halkidiki last night leaving 6 people dead and over 20 injured as trees toppled, power was knocked out and roads were blocked. There were also widespread power cuts.
"From now on, these phenomena will occur with increasing frequency, especially in the Mediterranean area which is sensitive to climate change", Efthymis Lekkas, a professor at Athens University's Department of Geology and Geo-environment, who heads a public agency for quake and disaster planning, told state-run TV.