Emergency services battled wildfires across swathes of southern Europe amid mass evacuations on Wednesday, as warnings sounded in London after Britain's hottest day that the fight against climate change needed to be stepped up.

Hundreds fled in central Italy as gas tanks exploded in a forest fire near the Tuscan town of Lucca. Similar numbers fled in Greece as a blaze fuelled by gale-force winds raged in mountains north of Athens. Greek authorities said later in the day that the blaze had been tamed.

A brutal heatwave with spikes well above 40 Celsius (104F) settled over southern Europe last week, part of a global pattern of rising temperatures, widely attributed by scientists and climatologists to human activity. It is forecast to dump searing heat on much of China into late August.

It is also expected to expose around 100 million Americans to temperatures above 38C on Wednesday and set records in Texas, Louisiana and Arkansas.

Power consumption is predicted to hit new peaks in Central U.S. states - adding to greenhouse gas emissions - as homes and businesses crank up air conditioners to combat a heatwave forecast to last into next week. 

While the record heat last week around parts of the Mediterranean has eased, mercury readings have begun heading up again in Portugal, Spain and Italy.

Armando Silva, Civil Protection commander in Portugal's northern region, said rising temperatures and strong winds would make it harder to fight the country's largest wildfire centred on the municipality of Murça.