Tennis players at Wimbledon, fasting Muslims and ailing religious pilgrims in Lourdes were among those at risk from the blast of a major heatwave spreading up through Europe on Tuesday.

With temperatures hitting 35 degrees Celsius (95 degrees Fahrenheit) in London, tennis players and fans were set for a very sticky Wimbledon tournament, which got underway Monday and is set to be the hottest on record.

Organisers have cut capacity and could suspend play if temperatures stay high.

A sweat-soaked seven-time champion Roger Federer said after his match that he was off to take a shower.

"I need one," he said.

"Actually I'm wet already so I'm not sure if I need one!" he then added.

In Belgium, where temperatures are heading into the high 30s this week, a passerby was forced to smash the window of a parked car to rescue a 13-month-old child that had been locked inside for over an hour, emergency services said.

In Lourdes in neighbouring France, millions of particularly vulnerable religious pilgrims hoping to be cured of illnesses had their daily ceremony on the main esplanade moved underground out of the blazing heat.

England declared a Level 2 health alert, and authorities raised concerns about Muslims fasting during daylight hours for Ramadan.

Fearful that railway tracks would buckle under the heat, Britain's Network Rail ordered trains to slow down.

Animal rights charities also put out a national appeal "warning people of the devastating consequences of leaving dogs in hot cars."

The Royal Society for the Protection of Animals said it had already received 96 calls last weekend for animal heat exposure.

- Ex-pope heads for hills -

For a continent unaccustomed to such warmth, the threats go beyond embarrassing wet patches on shirts and terrible body odour on commuter trains -- a heatwave in 2003 caused tens of thousands of deaths above the normal rate across Europe.

From Holland to Italy, governments warned of the risks to older people, young children and those with serious illnesses.

Former pope Benedict, 88, headed for the hills of Castel Gandolfo south of Rome to escape "il grande caldo" which threatened temperatures of 37 degrees in Rome and 40 in Sardinia.

Civil servants in the Belgian region of Wallonia had their hours cut so they would not have to swelter in traffic jams.

British trade unions said workers should be allowed to ditch tights, ties and suits wherever possible.

Authorities in Brussels were also preparing to cancel all major public and sporting events should the mercury remain high or pollution levels hit dangerous levels -- a measure introduced after the 2003 heatwave but so far never used.

Spain saw scorching temperatures as high as 44 degrees on Monday, triggering a red alert, the highest level on the scale, from the national weather office for the region of Cordoba, meaning the weather posed an "extreme risk" to health.

Many took the opportunity to flock to beaches across the Iberian peninsula.

"These are not usual meteorological phenomena, (they are) of an exceptional intensity and with a very high level of risk for the population," a spokesman for Spain's national weather office said.

France's weather office put 40 regions on orange alert, warning of an "enduring heatwave of significant intensity requiring particular vigilance".

Swiss authorities said the hot weather could last more than seven days, while Nordic countries were still enjoying relatively cool temperatures as they waited for the heat to reach them later this week.

burs-er/jm