Frantic relatives are trying to track down people missing and Some people appeared on television to plead for help.
Some 187 people were hospitalised, with 71 still being treated as of Wednesday evening, including nearly a dozen children, most of whom were in a "serious condition", the fire services said.
A number of roads and evacuation routes were blocked by fire and footage of motorists escaping the area shows them driving through thick smoke.
The seaside resort of Mati, in Attica, is popular with local tourists, especially pensioners and children attending holiday camps.
Dozens of forest fires sprang up on Monday, the deadliest of which started near Athens and sent citizens running into the sea to try and escape the inferno.
Hundreds fled to nearby beaches, with many swimming out to sea to escape the ferocious flames and choking smoke that killed 81 people.
"We are dealing with something completely asymmetric", Mr Tsipras said.
Merchant Marine deputy minister Nektarios Santorinios, whose ministry is in charge of the coast guard, said more than 700 people had been evacuated by sea by the coast guard.
"Fifteen fires had started simultaneously on three different fronts in Athens", said government spokesman Dimitris Tzanakopoulos.
As the U.S. State Department noted, major fires also have broken out in the area northeast of Athens (Penteli, Viotza, Mati, Rafina, and Agios Andreas), as the country suffers through extremely dry and hot weather, following a dry winter.
Authorities said Cyprus and Spain offered assistance after the request for European Union help was made.
The worse affected area was the seaside community of Mati, where the majority of victims were found, including 26 people found huddled together, many embracing.
Fire officials said several fires seemed to erupt at once in the area outside the capital of Athens, where temperatures had been in the 90s for days. The service had also received dozens of calls reporting missing people, but it was unclear if some of them were among those already found dead, a spokesperson said.
"It will be very painful, we will then have the identification ... the funerals, more pain", said Evangelos Bournous, the mayor of Rafina-Pikermi, an area close to Mati.
Supreme court prosecutors announced they had opened an investigation into the causes of the fire.
"It reminded me of the eruption at Pompeii - exactly that - and I said, 'My God, we must run to save ourselves and nothing else.' It was awful".
One of the youngest victims was thought to be a six-month-old baby who died of smoke inhalation, officials said.
Greece has also sought global help through the European Union and Spain is sending two firefighting aircraft while Cyprus has sent in 60 firefighters.
The losses are the most devastating for the country since 2007, when more than 60 people were killed by fires that swept through Greece's Peloponnese region.
A member of a rescue team searches inside a destroyed house following a wildfire at the village of Mati, near Athens, Greece, July 25, 2018.
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It is hoped that heavy rain forecast across southern Greecewill boost the firefighting effort.
Fires are not uncommon in Greece's forests, which cover nearly half its territory, and the Mount Penteli area has suffered regularly in the past decades.