More than 3,500 people had been evacuated by early Tuesday, some abandoning their cars on foot, but around 1,200 cars remained stuck on the Attiki Odos, the capital's main ring-road, government spokesperson Giannis Oikonomou said.
"We had a very difficult night and a superhuman evacuation effort is underway on Attiki Odos," Oikonomou told Greek television.
"We are still in a very difficult phase, as the forecasts indicate that we will face (weather) difficulties again in a while," he said.
As temperatures fell overnight, soldiers handed out food, water and blankets to drivers, some of whom were stranded in their cars for more than 10 hours. TV footage showed the road and vehicles covered in snow.
Authorities declared Tuesday a public holiday, shutting public offices and private businesses except supermarkets, pharmacies and petrol stations in the greater Athens area and on some islands.
Parts of Athens were hit by power cuts, and the grid operator said crews were working to restore electricity.
Fifteen passengers were injured when a rail transport vehicle tried to pull a train carrying about 200 passengers which had halted in heavy snow in central Greece.
The storm, named Elpida, was forecast to persist until Wednesday.