"We are a bit disappointed," said Chistelle Lasser from Belgium. "You come specifically for this, and it's closed."
Some visitors resorted to taking pictures of the ancient citadel through the closed metal gate.
"There's always YouTube," she told her dejected 14-year-old daughter.
The culture ministry had only announced the emergency shutdown of the 2,400-year-old Parthenon hours earlier.
Tourists were barred from that site and other ancient masterpieces atop the UNESCO-listed archaeological site between noon and 5:00 pm local time (0900-1400 GMT).
The closure -- the second in just over a month -- comes as Greece's national weather service predicted temperatures above 43 degrees Celsius (109 degrees Fahrenheit) in parts of the country.
It was the tenth consecutive day with temperatures over 40 degrees Celsius in some areas.
"It's absolutely boiling," said 25-year-old Lucy Johnson, who is on a cruise that docked at the Athens port of Piraeus.
"I'm not used to this weather: in the UK it's normally raining," she told AFP.
Red Cross mobilised
Sipping a soft drink in the shade, Brazilian Diana Bittai said she could not fault Greek authorities for the move.
"I think it's right, it's too hot," admitted the 49-year-old, who lives in London and was visiting with her sister.
Authorities had already closed down the Acropolis during the hottest hours of the day for two straight days in June, when Greece experienced its earliest-ever heatwave.
The Hellenic Red Cross was at hand to offer assistance, noting that they had handed out 5,000 bottles of water in the space of two hours.
"May I give you some information on heatwave, to protect you on these difficult days?" one Red Cross volunteer said while handing out information brochures.
A record number of almost four million visitors flocked to the Acropolis in 2023.
Barcelona-based Carlos Perez, 53, had not brought a hat, unlike his wife who wore a cap as mid-morning temperatures on the Acropolis hit 33 degrees Celsius.
"She's the typical girl, I'm the typical man," he shrugged. "I didn't do anything. She did everything for (herself)."
Heatwaves getting worse
Greek heatwaves are getting worse, said Red Cross nurse Vasiliki Dalla.
"Every year is worse than the previous one. And the heat waves are (getting) stronger and longer.
"The people that are coming here, maybe they're coming from countries that (have) never experienced such kinds of heatwaves."
"Sometimes they overestimate their (strength)," Dalla said.
Several hikers have died in Greece this year in incidents attributed to high temperatures and a lack of caution.
One victim was British health guru and TV personality Michael Mosley, found dead on June 9 on the Greek island of Symi, days after he went missing on a walk back to his hotel.
Two elderly French women who disappeared while on a hike on the island of Sikinos on June 14 are still unaccounted for.
