Firefighters said they were investigating whether the bodies, found near a shack south of the village of Avantas, were migrants. The surrounding Evros region is a popular route for migrants from the Middle East and Asia crossing from Turkey.
In the Greek port town of Alexandroupolis nearby, dozens of hospital patients were evacuated onto a ferry, while a blaze on the foothills of Mount Parnitha sent thick clouds of smoke over the capital Athens.
In Spain, Italy and Portugal, firefighters were battling blazes as the region suffered hot, dry and windy conditions that scientists have linked to climate change.
Temperatures in many areas were expected to reach or exceed 40degC, forecasters said. Italy and France declared red alerts in a number of areas.
The latest heatwave comes after a torrid July that was the hottest month on record. Some 20,000 people were evacuated on the Greek island of Rhodes in mid-July and a severe fire hit Spain's Canary island of La Palma.
Blazes on Hawaii's Maui island earlier this month killed more than 110 people, while Canada this week deployed the military in its westernmost province of British Columbia to tackle fast-spreading fires.
In Greece, gale-force winds complicated efforts to control the fires. Fifty-six firefighters arrived in Greece from Romania on Tuesday and Athens was expecting further assistance from the Czech Republic, Croatia, Germany and Sweden.
The 18 bodies were found south of the village of Avantas near the vast Dadia forest, authorities said. Another body thought to belong to a migrant was found on Monday, in a rural area some 40km away.
"The possibility that these are people who entered the country illegally is being investigated," the fire brigade said. It said searches were ongoing.
In Alexandroupolis, not far from Avantas, wildfires forced the evacuation of dozens of hospital patients, including newborn babies. A ferry was turned into a makeshift hospital after 65 patients were evacuated from the University Hospital.
"I've been working for 27 years, I've never seen anything like this," said nurse Nikos Gioktsidis. "Stretchers everywhere, patients here, IV drips there ... it was like a war, like a bomb had exploded."
Fires also broke out on Tuesday near Athens, where a blaze on the city's outskirts, on the foothills of Mount Parnitha, burned homes and forced residents to flee.
"The winds are very strong ... It is a very difficult firefighting task. God help us," said Sotiris Masouris, a 50-year-old resident of Hasia, west of Athens.
