Authorities issued warnings urging children and the elderly to stay indoors as a week-long heatwave kicked in across Europe.

The temperature had already reached 31 degrees Celsius early in the morning in Paris and Rome with high levels of humidity.

A tourist in Rome tries to keep cool. ( AFP: Alberto Pizzoli )

Meteorologists blamed a blast of torrid air from the Sahara for the unusually early summer heatwave, which could send thermometers up to 40C in some places on Thursday and Friday.

"That means extreme thermal stress. And that means being very cautious, avoid physical activities during the day, be in the shade, and of course drink a lot of liquids," said meteorologist Andreas Friedrich.

In Paris, locals and tourists stripped down to paddle in the Trocadero fountains by the Eiffel Tower. ( AP: Alessandra Tarantino )

In Berlin, authorities warned the temperature could reach 40C in the coming days, as zookeepers at Berlin's zoo gave its elephants refreshing showers to cope with the heat.

Zookeepers fed a group meerkats frozen treats — ice blocks with worms inside — while the orangutans played with a hose to cool off.

Lemurs snacked on fruit icicles to beat the heat. ( AP: Andrew Medichini )

Other animals including bears and lemurs were fed frozen snacks with fruit pieces inside at various zoos across Europe.

Tourists in Rome and Paris scaled back their sightseeing in sweltering temperatures on Tuesday, dousing themselves at fountains and cooling down with ice cream.

Tourists in Rome scaled back sightseeing to douse themselves at fountains. ( AP: Alvaro Barrientos )

"With this hot sun we try and keep to the cooler places," said Fari, a Spanish tourist at Rome's Trevi Fountain.

"From 8 o'clock we are out looking at Rome, and from midday we eat ice cream," he added.

The French health minister said more than half of France is on alert for high temperatures. ( AFP: Manuel Cohen )

In Paris, locals and tourists stripped down to paddle in the Trocadero fountains by the Eiffel Tower.

"It's really, really uncomfortable right now. The heat is really high, I think it's over 35 degrees. So for us, doing a tour of two hours and a half, three hours, is really, really difficult," said 32-year-old Argentinean tour guide to the city Ayelen Rozitchner.

Temperatures across the continent could reach 40 degrees Celsius in some places on Thursday and Friday. ( AP: Jens Meyer )

French forecaster Meteo France issued an orange alert, the second-highest level of weather warning, projecting highs of 34C in the French capital, and up to 39C elsewhere.

On Monday the government announced it would suspend exams sat by 14 and 15-year-olds, prompting accusations from the opposition that it was overreacting.

Orangutans at the Schoenbrunn zoo in Vienna filled cups with water to drink in the heat. ( AP: Daniel Zupanc/Vienna Zoo )

"This isn't scaremongering," Health Minister Agnes Buzyn told a news conference.

"I'm asking everyone to take responsibility for themselves, their family and their neighbours and to avoid a backlog in emergency rooms due to people taking unnecessary risks."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 35 seconds 35 s French authorities warn against dangers of extreme heat

The heatwave has revived memories in France of August 2003, when the searing heat overwhelmed hospitals and caused the deaths of some 15,000 people, mostly the elderly.

The government says much progress has been made since. Authorities in Paris are setting up "cool rooms" in municipal buildings, opening pools for late-night swimming and installing extra drinking fountains.

Polar bear Nanook bathed to cool down at the Gelsenkirchen zoo in Germany. ( AP: Roland Weihrauch )

In Poland, the ongoing heatwave has created a spike in demand for ice cubes.

Workers at a huge factory in the west of the country are now trying to meet increased orders in one of the coolest places in sweltering Europe.

Germany expects temperatures to get hotter in the coming days. ( AP: Michael Probst )

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