Greece has never seen anything close to the record levels of rain that have battered the country over the past few days, ITV News Correspondent Neil Connery reports
There have been days of heavy rain, covering towns and villages across the region in mud and turning roads to rivers.
The city of Volos recorded London's annual rainfall in 24 hours, just a few weeks after enduring wildfires.
"From Volos' burning to Volos' drowning," one woman told ITV News.
"People are drowning and homes lost."
Greek meteorologists have called the floods, the "worst weather phenomenon in a century".
Five people were killed in Turkey when the campsite they were staying in was flooded. One person remains missing.
Another two people died in Istanbul, Turkey's largest city, where Tuesday's storms inundated hundreds of homes and workplaces.
The victims in Istanbul included a 32-year-old Guinean citizen, who was trapped inside his basement apartment.
A 57-year-old woman died after being swept away by the floods in another neighbourhood, Turkish news reports said.
The surging floodwaters affected more than 1,750 homes and businesses in the city, according to the Istanbul governor's office.
They included a line of shops in the Ikitelli district, where the deluge dragged parked vehicles and mud into furniture stores, destroying merchandise.
The floods also engulfed a parking area for containers and lorries on the city's outskirts, where people found safety by climbing on top of the roof of a restaurant.
Widespread flooding in central Greece has left at least four people dead, six missing and dozens trapped, with severe rainstorms turning streams into raging torrents, bursting dams, washing away roads and bridges, and hurling cars into the sea.
In some parts of Greece, a year's worth of rain fell in just one day - channeling thigh-high torrents through streets and sweeping cars away - as the country faced more extreme weather following a summer of wildfires.
A man was killed near the central city of Volos when a wall buckled and fell on him.
Three people were reported missing.
Authorities banned traffic in Volos, the nearby mountain region of Pilion and the resort island of Skiathos, where many households remained without electricity and running water on Wednesday.
Vassilis Kikilias, Greece's minister for climate crisis and civil protection, said more than 885 people had been rescued so far and six were reported missing.
British tourists were among those stranded as three regions of Greece were hit by Storm Daniel, causing widespread flooding that turned roads into rivers.
Isabella Smith, who is on holiday on Skiathos, compared weather conditions there as something akin to a "warzone".
She told ITV News: "I've never seen anything like it. The whole sky was kind of just lit up frequently, the hotel room... obviously it was dark, but it kept kind of the whole room would light up and the noise was just kind of… yeah it was just absolutely terrifying."
'The noises felt like it was a warzone,' Isabella Smith describes to ITV News the weather conditions she has experienced while on holiday on Skiathos
Four flights scheduled from the UK to the Island on Wednesday have also been cancelled.
On Monday, a flight from London Stansted to Skiathos was diverted to the mainland city of Thessaloniki because of the weather, where customers were provided with hotel accommodation until they can be returned to the UK.
In a statement to affected customers posted on its website, Jet2 said: "We are sorry to inform you that due to the current and forecasted weather affecting the Island of Skiathos, we have delayed your flight.
"The heavy rainfall and thunderstorms are continuing to effect Skiathos, this is part of an adverse weather phenomenon named Storm Daniel.
"We are monitoring the forecast and will confirm your new departure time as soon as the forecast allows.
"We understand how frustrating this situation is, but please be assured that our dedicated UK-based Operations Team are working tirelessly to ensure we get you back to the UK, when it is safe to do so.
"We know that this is not the news you would like to hear, however, the safety and well-being of all our customers and crew is paramount. Whilst in destination, please continue to follow the advice of the Local Authority."
In Bulgaria, a storm caused floods on the country’s southern Black Sea coast, and the bodies of two missing people were recovered from the sea on Wednesday, raising the overall death toll to four.
Video showed cars and camper vans being swept out to sea in the southern resort town of Tsarevo, where authorities declared a state of emergency.
Footage captures the moment that floodwaters swept cars into the sea in Bulgaria
Most of the rivers in the region burst their banks and several bridges were destroyed, causing serious traffic problems.
Tourism Minister Zaritsa Dinkova said that about 4,000 people were affected by the disaster along the entire southern stretch of Bulgaria’s Black Sea coast.
“There is a problem transporting tourists because it is dangerous to go by coach on the roads affected by the floods,” she added.
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