Fast-moving flames, whipped up by 50mph winds, trapped families as they tried to escape Rafina, a seaside region 18 miles east of the capital on Tuesday.
But the number of victims is set to rise with rescue crews forced to check charred homes and vehicles for bodies.
Rescuers also scoured beaches and deeper waters for survivors who sought to escape the fires, despite rough conditions at sea.
The fires – one to the west of Athens near the town of Kineta and another to the northeast near the port of Rafina – spread at a speed that surprised many, trapping hundreds on beaches and cutting off escape routes.
They broke out on Monday afternoon during a hot, dry spell but the cause was not immediately clear. Aerial photos showed swathes of charred forests and homes.
Apart from the dead, which included children, hospitals treated 187 people, mostly for burns – with 10 in a serious condition.
Alexis Tsipras, the Greek prime minister, declared three days of national mourning on Tuesday.
“The country is going through an unspeakable tragedy,” he said in a televised address. “Today, Greece is mourning and we are declaring three days of national mourning in the memory of those who perished.”
Greece has called for international help through the European Union amid the growing disaster, as authorities deployed the country’s entire fleet of water-dropping planes and helicopters to give tourists time to escape.
The Greek Red Cross said on Tuesday it had found the bodies of 26 people at a pair of villas in the village of Mati, in Rafina.
The fire in Mati is by far the country’s worst since flames devastated the southern Peloponnese peninsula in August 2007, killing dozens.
“Mati doesn’t even exist as a settlement anymore,” one woman told Greece’s Skai TV. “I saw corpses, burned-out cars. I feel lucky to be alive.”
Mati is a popular retreat with local tourists, particularly pensioners and children at holiday camps.
Hundreds of people scrambled to the sea to be evacuated by navy vessels, yachts and fishing boats as the fire closed in.
Athens has issued an urgent appeal for help to tackle several fires that are still out of control, destroying property and disrupting major transport links.
Authorities said they would be making use of an unmanned drone from the US on Tuesday to monitor and track any suspicious activity.
Mr 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.
In a blaze earlier on Monday, Greek authorities urged residents of a coastal region west of Athens to abandon their homes as a wildfire took hold, closing one of Greece’s busiest motorways, halting train links and sending plumes of smoke over the capital.
Showers that passed over Athens on Monday missed the two biggest ongoing fires, one in Rafina and another at Kineta, 35 miles to the west of the city.
Although it had abated by Tuesday afternoon, the blaze was far from extinguished and more than 230 firefighters were still trying to put it out, helped by volunteers and water-dropping aircraft. Another five fires continued to burn, with flare-ups reported in the blaze near Kineta. Authorities ordered the evacuation of some communities as a preventive measure.
Authorities urged the public to contact them about the missing. Many took to social media, posting photos and what was believed to be their last location before the fires hit.
Red Cross rescuers said they appeared to be families or groups of friends because they were found hugging in groups of threes and fours.
Hundreds of homes and cars were believed to have been burned. Many vehicles were found with the keys still in the ignition and doors open, a sign of the urgency with which their occupants sought to flee the flames. Narrow roads quickly became jammed, forcing many to try to escape on foot. The ferocity of the fire melted cars’ metal hub caps.
Coast guard and private boats picked up more than 700 survivors from beaches and the sea – but also recovered six bodies.
“It happened very fast. The fire was in the distance, then sparks from the fire reached us. Then the fire was all around us,” said Nikos Stavrinidis, who had gone with his wife to fix up his summer home for a visit by his daughter.
Mr Stavrinidis, his wife and four friends swam out to sea to escape the smoke, but they quickly became disoriented, losing sight of shore and being swept out farther by the wind and currents. Two of his group didn’t survive.
“It is terrible to see the person next to you drowning and not being able to help him,” Mr Stavrinidis said, his voice breaking. The rest of the survivors were picked up by a fishing boat with an Egyptian crew who jumped into the water to rescue them.
Rafina’s dock became a makeshift hospital overnight as paramedics examined survivors, some wearing only their bathing suits, after being dropped off by rescue boats.
Evangelos Bournous, Rafina‘s mayor, said his home had burned down and his family escaped by going into the sea.
“Everything happened in seconds,” said Andreaas Passios, who lives next to the compound in Mati where the 26 bodies were found. “I grabbed a beach towel. It saved my life. I soaked it, grabbed my wife and we ran to the sea.”
Mr Passios said he and his wife stayed by the sea for two hours.
“It was unbelievable. Gas canisters were exploding. Burning pine cones were flying everywhere,” he said.
Among the survivors was former Greek Communist Party leader Aleka Papariga.
“The police tried to direct us away from the fire, but we couldn’t escape it,” she said. “We got stuck in traffic and the flames were on top of us. We managed to find a small gap and we made it out.”
Local officials provided housing, food and clothes for those affected.
Greece sought help in fighting the fires from the European Union. Spain sent two firefighting aircraft, while Cyprus sent in 60 firefighters. Israel, Turkey, Macedonia, Bulgaria, Croatia, Italy and Germany also offered assistance.
Over the two days, 47 brush and forest fires broke out across Greece, with most of them quickly extinguished, the fire department said.
Heavy rain was forecast Wednesday across southern Greece, and there was hope that could help firefighters.
Forest fires are common during Greece’s hot, dry summers and temperatures recently reached up to 40C.
Additional reporting by agencies
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