A wildfire killed at least 50 people and injured more than 150 as it swept through a small resort town near Athens, with huge flames trapping families with children as they fled.
The fire which hit Mati, 29 km (18 miles) east of the capital, late Monday afternoon was by far the country's worst since flames devastated the southern Peloponnese peninsula in August 2007, killing dozens.
People scrambled to the sea as the blaze closed in close to the shore. Hundreds were rescued by passing boats but others found their way blocked by smoke and flames.
"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," Nikos Economopoulos, head of Greece's Red Cross, told Skai TV.
"They had tried to find an escape route but unfortunately these people and their kids didn't make it in time," he said. A Reuters witness also saw several bodies in the area.
Mati is in the eastern Rafina region, a popular spot for Greek holidaymakers, particularly pensioners and children at camps.
The Israeli Foreign Ministry issued a statement on Facebook, following the raging wildfire: "In light of the unusually severe fires raging in Greece, Israeli authorities (the National Security Council in the Prime Minister's Office and contacted their counterparts in Greece and offered Israel's aid. Greek authorities responded that at this stage they have gained control of the fires. Israel has emergency teams waiting on stand-by.
"Israel stands ready to aid Greece in any way necessary. The Greek authorities have expressed their thanks to Israel for its offer of aid."
The 26 deaths came on top of more than 20 casualties reported by government spokesman Dimitris Tzanakopoulos earlier on Tuesday. He said more than 88 adults and 16 children were injured.
One of the youngest victims was thought to be a six-month-old baby who died of smoke inhalation.
A Reuters witness earlier saw at least four dead on a narrow road clogged with cars heading to a nearby beach.
The coastguard later said the bodies of four more people were retrieved from the sea. In total, coastguard and other vessels rescued 696 people who had fled to beaches. Boats plucked another 19 people from the sea.
Some parts of Mati were still smouldering with white smoke early on Tuesday. Burned-out cars were scattered outside gated compounds where three- and four-storey buildings bore signs of fire damage.
"We are dealing with something completely asymmetric," Greek Prime Minister Alexis Tsipras, looking pale, said after cutting short a visit to Bosnia.
Greece issued an urgent appeal for help to tackle fires that raged out of control in several places across the country, destroying homes and disrupting major transport links.
Cyprus and Spain offered assistance after Greece said it needed air and land assets from European Union partners.
The inferno dominated front pages in the country on Tuesday, with headlines such as "killer fire" and "hell" and newspapers reporting fears the death toll would climb.
Authorities said they would be making use of an unmanned drone from the United States on Tuesday to monitor and track any suspicious activity.
Tsipras and Greek officials have expressed misgivings at the fact that several major fires broke out at the same time.
Wildfires are not uncommon in Greece, and a relatively dry winter helped create current tinder-box conditions. It was not immediately clear what ignited the fires.
A hillside of homes was gutted by flames east of Athens. A mayor said he saw at least 100 homes and 200 vehicles burning.
Earlier on Monday, Greek authorities urged residents of a coastal region west of Athens to abandon their homes as another wildfire burned ferociously, closing one of Greece's busiest motorways, halting train links and sending plumes of smoke over the capital.
The main Athens-Corinth motorway, one of two road routes to the Peloponnese peninsula, was closed and train services were cancelled.