An estimated 23 children were injured in the fire, which first struck Mati on Monday afternoon, a town 18 miles east of the Greek capital.
The fire has been declared the worst in Greece since the 2007 southern Peloponnese peninsula, which killed approximately 70 people and injured hundreds.
As the fire engulfed the city, hundreds rushed to the sea to be rescued by passing boats, but several were unable to escape due to thick plumes of smoking and raging widespread fires.
Boats and helicopters were deployed to evacuate dozens of people from a resort beach, which was engulfed in flames.
Nikos Economopoulos, head of Greece’s Red Cross, told Skai TV: “I was briefed by a rescuer that he saw the shocking picture of 26 people tightly huddled in a field some 30 metres from the beach.
“They had tried to find an escape route but unfortunately these people and their kids didn’t make it in time.”
Speaking to Reuters, a witness said they saw several dead bodies around them.
Greek Prime Minister Alexis Tsipras declared three days of national mourning for those killed in the fires.
He said: “We will do whatever is humanly possible to control it. I’m concerned at the simultaneous outbreaks both east and west of Attica.
“Instinctively, seeing the end nearing, they embraced.”
The fire in Mati – a popular tourist destination and home to holiday villas and resorts – was caused by an intense heatwave in Greece, following weeks of sizzling temperatures.
Authorities declared a state of emergency in the eastern and western parts of greater Athens, as fires spread through pine forests and seaside towns across the capital.
Responding to Greece’s plea, Cyprus has sent several firefighters while Spanish aircrafts have been deployed to extinguish flames.
Mr Tsipras added: “We are dealing with something completely asymmetric.”
Officials are also investigating the whereabouts of 10 missing people, who are believed to have fled the fire on a sail boat.
Fire brigade spokeswoman Stavroula Maliri said: “Residents and visitors in the area did not escape in time even though they were a few metres from the sea or in their homes.”
Pantelis Saitas, a photographer for the Athens-Macedonian News Agency, said: “There is a pile of 26 bodies, just 15 meters from the sea. Police forces have blocked the area.
“These people must have tried to find a way out into the sea but they were likely trapped in the flames.”
Yesterday, government spokesman Dimitris Tzanakopoulos reported that 11 people have been held in intensive care.
A spokesperson for the Association of British Travel Agents (ABTA) said: “The wildfires in Greece are northeast of Athens in the Penteli area and 50km west of Athens.
“This area isn’t a main destination for UK tourists. People on holiday in the region are advised to monitor news reports and follow any advice or instructions issued by the local authorities, their travel company or their accommodation providers.
“Anyone due to travel imminently to the areas affected should speak to their travel company or accommodation provider.”