The tremor measuring 6.5 on the Richter scale struck 54 kilometers (33 miles) south of the western port city of Patras on Sunday, injuring 125, destroying homes.
Hundreds of villagers spent the night in tents, in cars or sleeping bags in town squares, too scared to return to their homes, as aftershocks continued through the night.
"We are destroyed," said a resident of the village of Valmi, one of the hardest hit, who did not want to be named. "The quake was so strong that even graves opened up."
A Reuters photographer said more than half of the village's 46 homes had collapsed.
"There is a tent wherever there is space," said Reuters photographer Yiorgos Karahalis. "Shopowners are cleaning up smashed glass to get their businesses going again and trying to get their lives back on track."
Tents for the homeless
Three hundred tents were sent to the region for about 250 people who are estimated to have been left homeless.
In the village of Kato Achaia, also hard hit by the quake, one man was killed when a building collapsed and an elderly woman who was taken to hospital with injuries died of a heart attack.
Authorities said most of those hospitalised had minor injuries. There were no foreigners among them.
One 9-year-old girl was among a handful of people who were trapped in collapsed buildings and rescued soon after the quake.
"Aftershocks will continue for months. We are expecting aftershocks measuring 5-5.5 on the Richter scale," an official at the Athens Geodynamical Institute told Reuters.
Greek emergency services have sent more than 200 firemen to the region, while the government has offered immediate cash assistance of 3,000 euros to those whose houses have been hit.
Seismologists warned that a strong aftershock was expected in southwestern Greece a day after a powerful earthquake.
Güncelleme Tarihi: 09 Haziran 2008, 18:18
But ''the next two to three 24-hour periods remain critical,'' said Giorgos Stavrakakis, head of the Athens Geodynamic Institute.
A strong aftershock of a magnitude of about 5 or 5.5 was still expected in the coming days or possibly weeks, seismologist Giannis Kalogeras said.
''An earthquake of 6.5 in western Greece has a rich post-seismic activity,'' he said. ''An earthquake of 5 or 5.5 is expected. That is based on our experience of the region.''
Prime Minister Costas Karamanlis was cutting short a visit to Austria to return to Athens on Monday.
Sunday's quake was felt in Athens and as far away as Italy. In 1999, a deadly tremor measuring 5.9 on the Richter scale killed 143 people in the Greek capital. Greece is often rattled by quakes but most do not cause serious damage.