Europe, US sizzled in a heatwave yesterday, with London bracing for Britain's hottest June day since 1976 as Portugal battled to stamp out deadly forest fires.

More than 1,000 firefighters were still fighting to control the flames that broke out in central Portugal at the weekend, killing 64 people.

Cooler weather was aiding their efforts, but thermometers were still hovering around 35 degrees Celsius (95 degrees Fahrenheit) -- a level matched across oven-like swathes of Europe, including Italy, Austria, the Netherlands and even alpine Switzerland.

Two forest fires have also broken out since Sunday on Croatia's southern Adriatic coast, prompting the authorities to evacuate 800 tourists, though the blazes have now been brought under control.

"We were scared, it's true," a Swedish tourist who gave her name only as Karolina told local media. "From the hotel room window it seemed like the fire was at our doors."

As the northern hemisphere marked the summer solstice, firefighters were on alert in Austria, where revellers were due to celebrate the longest day of the year by lighting bonfires for the festival of Saint John.

In Italy, forecasters say the current heatwave could turn out to be the most intense in 15 years, with temperatures around eight degrees above the seasonal average -- 39 degrees Celsius in Milan and up to 30 in the Alps at an altitude of 1,000 metres (3,300 feet).

Britain was set to see its first five-day stretch of temperatures over 30 degrees in June since 1995. The thermometer was forecast to hit 34 Celsius in west London, a record for the month since 1976.

In Guildford, southwest of the capital, a road surface melted on Tuesday, with motorists likening it to a bar of chocolate left out in the hot sun.

A scorching heatwave in the western United States has grounded flights, caused fires and prompted power outages, with record temperatures expected Tuesday in several states including Arizona and Nevada.

The extreme temperatures prompted American Airlines on Tuesday to ground 43 flights to and from Phoenix Sky Harbor Airport, where temperatures were set to reach a record 120 degrees Fahrenheit (49 degrees Celsius).

In California, the National Weather Service issued an excessive heat warning for southern parts of the state including the cities of Banning and Desert Hot Springs, where temperatures could soar as high as 122 degrees.

Officials said the heat has led to electricity outages, as more people turn on air conditioners, overtaxing the power system.

Several wildfires were also burning in the state, including a massive one in the San Bernardino National Forest, east of Los Angeles, where 450 firefighters were battling the blaze that has already scorched 950 acres.

Authorities urged people to stay indoors and keep hydrated to avoid any heat-related illnesses.