Damaged umbrellas and lounge chairs on a beach after a storm in Chalkidiki. (AFP)

Fierce storms have killed six tourists and injured dozens of people in northern Greece, authorities said on Thursday.

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.

"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.

"There was panic, people were howling and running to hide inside," Lazaridis told AFP. He added that over 100 people were sheltering under the roof when it collapsed.

Nearly 30 people have been hospitalised in nearby Thessaloniki, including a 13-year-old girl from Serbia.

"It was a miracle that there weren't more deaths," said 39-year-old Kyriakos Athanasiadis, who is vacationing in the area.

"Nearly all the coastal restaurants were full, and you could see large objects flying," he told AFP.

One woman reportedly told hospital staff she was picked up by the wind and thrown in a garbage bin, which then rolled away.

The freak storm only lasted about 20 minutes, but it was enough to overturn cars, uproot trees and balcony railings and cause mudslides.

"Unprecedented Phenomenon"

On a beach in Sozopoli, the storm toppled and ripped open a Czech family's caravan, killing an elderly couple in their seventies and injuring their 48-year-old son and 19-year-old grandson.

Elsewhere in the region, a Russian man and his son were killed by a falling tree.

"It was an unprecedented phenomenon," said Charalambos Steriadis, head of civil protection in northern Greece.

Officials have declared a state of emergency and army crews were working around the clock to restore electricity.

"I want to express my sorrow on behalf of all... We mourn for the loss of these souls," said Citizen Protection Minister Michalis Chrisochoidis, who is overseeing operations in the area.

"We are in solidarity with their relatives, with the people who have lost their families," he added.

According to port police, a fisherman in his sixties was also missing.

At least 140 rescue workers were involved in the operation, emergency chief Vassilis Varthakoyannis said.

Prime Minister Kyriakos Mitsotakis, who took over Sunday after general elections, cancelled his meetings to address the disaster, his office said.

The storms came after temperatures in Greece soared to 37 degrees centigrade (98 degrees Fahrenheit) over the past two days.

The Greek national observatory said it had recorded over 5,000 lightning bolts around the country on Wednesday evening.