More than half of France’s population is under a severe weather warning as large swathes of western Europe endure the second extreme heat event of the year with temperatures expected to exceed 40C (104F).

The French president, Emmanuel Macron, called for “extreme vigilance from everyone”, asking citizens to “take care of our oldest and most vulnerable people” and follow government advice. “We are going through difficult days,” he said.

A 30-year-old man died after going into cardiac arrest on an athletics track near Paris on Thursday as the temperature hit 37C, prompting the rail operator SNCF to cancel 71 intercity trains and schools to reschedule exams.

The national weather service, Météo-France, extended its ⁠orange heatwave alert to cover 53 of the country’s 96 mainland departments – home to 36 million people – from midday on Friday, warning of a “widespread, prolonged ​and intense” heatwave.

Astronomical summer does not begin until Sunday, but the heatwave is already France’s second extreme temperature event of the year after an unusually hot spell in May that shattered local and national monthly temperature records.

Météo-France said temperatures were likely to average 36C in the north-west and 38C in the centre and south on Friday. After a slight dip on Saturday they are forecast to rise to 40C in many regions, including Paris, early next week.

With so much of the country affected, the agency said the national heat index, an average of the night and daytime temperatures recorded at 30 weather stations nationwide, could approach all-time highs on Sunday and Monday.

The power utility EDF ​has said four nuclear ​plants ​were likely to curb output next week because of high cooling water temperatures in the Rhône and Garonne rivers, and several municipalities have cancelled Sunday’s Fête de la Musique festivities.

A spokesperson for Spain’s state meteorological office, Aemet, said temperatures would reach 40C as the country entered “an episode of persistently high temperatures likely to meet the technical threshold for a heatwave”.

Rubén del Campo said temperatures were likely to exceed 35C across the Iberian peninsular and Balearics, hitting 40C in some southern parts – such as the Tagus, Guadiana and Guadalquivir valleys – but also in eastern Cantabria and the Ebro valley in the north.

The heatwave could last until next Wednesday or Thursday, when temperatures are expected to fall, but it could remain “very hot” across much of the country, with night-time temperatures not falling below 25C in many areas.

Temperatures in south-west Germany are forecast to rise to 36C by the weekend, prompting authorities to issue heat warnings even at altitudes of 600 metres (2,000ft). The DWD weather service also forecast heavy thunderstorms and downpours.

The agency advised people to avoid physical activity where possible, whatever their level of fitness, and for non-swimmers to take care after a spate of recent drownings during hot spells.

A hitzefrei or heat-free day was declared for Friday, meaning lessons were cut short and pupils sent home early because buildings were heating up to unmanageable temperatures.