Tourists were unable to visit the Parthenon and other ancient masterpieces atop the UNESCO-listed archaeological site between noon and 5:00 pm local time (0900 to 1400 GMT).

Meteorologists have noted it is the earliest heatwave -- which for Greece means temperatures exceeding 38 degrees Celsius (100 degrees Fahrenheit) for at least three days -- on record.

In central Athens, the mercury climbed to 42 C, with the forecast heat prompting health warnings and school closures.

Still hotter temperatures were recorded on the island of Crete -- 44.5 C -- and on the Peloponnese peninsula -- 43.9 degrees Celsius --- according to the meteo.gr website of the Athens National Observatory.

Several other Greek archaeological sites in Crete, the Dodecanese, the Cyclades and the Peloponnese were also shut as a precaution against the heat.

Greece's climate crisis and civil protection minister Vassilis Kikilias warned of the risk of fire due to strong winds that could blow across the country. Civil protection was on "high alert", he added.

The risk of fire will be "very high" on Friday in ten regions, including in Attica, which surrounds Athens, his ministry also warned.

Temperatures are expected to ease from Friday onwards.

The Acropolis was forced to close in July last year during a two-week heatwave that was unprecedented in its duration.

It was followed by fires that, according to the National Observatory of Athens, consumed nearly 175,000 hectares (432,000 acres) of forest and farmland.

A record number of almost four million visitors flocked to the Acropolis in 2023, its popularity boosted in part due to tourists arriving on cruise ships calling in at the nearby port of Piraeus.