The UNESCO-listed archaeological site closed from midday to 5:00 pm (0900 to 1400 GMT), with temperatures topping 43 degrees Celsius (109 degrees Fahrenheit) in central Greece.
Temperatures of up to 44 degrees Celsius are expected on Thursday as the phenomenon peaks, with up to 43 degrees forecast in the capital.
Meteorologists have noted this is the earliest heatwave -- which for Greece is temperatures exceeding 38 degrees Celsius for at least three days -- on record.
"This heatwave will go down in history," meteorologist Panos Giannopoulos said on state TV ERT.
"In the 20th century we never had a heatwave before June 19. We have had several in the 21st century, but none before June 15," he said.
The climate crisis and civil protection ministry has warned of a very high risk of fires in the Attica region around Athens.
Schools stayed closed in several regions of the country on Wednesday and will do so again on Thursday, including in the capital, while the labour ministry has advised public-sector employees to work from home.
The ministry also ordered a pause from midday to 5:00 pm for outdoor work including food delivery.
Still, many employees are obliged to take their own precautions.
Tassos Konstantinidis, a 25-year-old selling mobile phone cards for a major telecoms company, wore a cap and brought along an icebox for his water.
"I lived this last year when we had almost 10 days of heatwave," he said.
"It's up to me to organise myself and take breaks if needed," he told AFP. "My employer has made no such provisions."
- 'Too risky' -
Sheltering under a parasol, electrician Fotis Pappous said he had started his workday a few hours earlier than usual, at 6:00 am, on orders from his employer.
"With this kind of heat, it would be too risky otherwise," said the 46-year-old as he tinkered with an electricity meter near Athens's central Syntagma Square.
But for staff working over the grill in Greece's already-buzzing tourist Monastiraki district, there was no room for respite.
"We have no choice, it's the start of the tourist season," said kebab store owner Elisavet Robou.
"We have air-conditioning and fans, and staff are allowed to take breaks, but unfortunately the climate crisis is here.
"Heatwaves came earlier this year and the season will be difficult," she said.
An air-conditioned hall has been opened at Syntagma metro station in central Athens to give people somewhere to shelter from the heat, the public transport authority said.
Greece's Red Cross said it had handed out some 12,000 bottles of water in the centre of the capital and at the Acropolis.
In Greece's second city Thessaloniki, teachers and pupils said annual school exams were held under difficult conditions.
"There was no air-conditioning in any of the rooms so we used fans, some of which the teachers brought from their own homes," said Andreas Karagiannis, a 52-year-old mathematician and examiner.
"Exams should not have been held under these conditions," said 17-year-old pupil Yiannis Theodoridis.
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 last year, with its popularity boosted in part due to tourists arriving on cruise ships calling in at the nearby port of Piraeus.
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