Greece's hottest day this year is forcing authorities to shut down the Athens Acropolis six hours before schedule in the interests of visitor health, the site's guards said on Monday.

The country's top monument was to shut down at 1100 GMT instead of its normal 1700 GMT closing time, a guard told AFP.

The ancient citadel is perched on a rocky plateau rising amid a sea of concrete in the Greek capital of over four million, offering precious little shade to thousands of tourists who visit it daily.

Temperatures in Athens were set to exceed 42 degrees Celsius (107.6 Fahrenheit) at the close of this year's hottest week that earlier forced authorities to make air-conditioned halls available to the public.

The environment ministry said air pollution was also above warning levels in various parts of the capital as it warned people with respiratory problems and heart trouble to stay indoors.