Greece's meteorological service said more than 5,000 bolts of lightning were recorded in northern Greece, with the storms dying out as they headed eastwards.
"It was so abrupt, and so sudden".
Elsewhere, two elderly Czech tourists were killed when strong winds and water swept away their travel trailer.
A woman and an eight-year-old boy from Romania were killed when a roof collapsed on a restaurant in the beach resort of Nea Plagia.
Strong winds and hail on Wednesday evening tore into the beachfront of Halkidiki region, one of Greece's most popular tourist areas, terrifying thousands of holidaymakers caught in the open.
Gale-force winds, heavy rain and hailstorms lashed Halkidiki, near the city of Thessaloniki, late on Wednesday, officials say.More news: Robot umpire used for the first time in pro baseball history
A man and a young boy, both Russians, died after a tree collapsed near a hotel in the seaside town of Potidea, authorities said.
Six tourists have been killed and dozens of people injured in fierce storms in northern Greece, authorities have said.
He said his patients ranged from 8 months in age to 70 years old, and some had suffered head injuries from falling objects.
Videos captured by local media showed the storm damage, including vehicles crushed by uprooted trees and debris, and roofs that had been ripped off buildings.
The freak storm lasted about 20 minutes, according to witnesses interviewed by the state broadcaster ERT.More news: 'First collective response': China rebuked by 22 countries for detaining Uighurs
At least 140 rescue workers were involved in the operation, emergency chief Vassilis Varthakoyannis said.
A state of emergency was put in place on the Halkidiki peninsula amid what has been described by meteorologists as "extremely unusual" conditions for the time of year.
Meteorologists forecast it would continue to rain in the area until about 9am (06:00 GMT) on Thursday.
Temperatures had soared to around 37C (98.6F) in the days leading up to the storm.
Citizen protection minister Mihalis Chrisochoidis told reporters during a visit to the scene his country was "mourning" the loss of life.More news: Firefighters Tackling Blaze at Warner Bros.' UK Studios at Leavesden