The French Health Minister Xavier Bertrand has urged medical students and retired doctors to help in hospitals during the current heatwave.
Bottled water is being rushed out to vulnerable people in France
The heat has now claimed 23 lives in France, reviving memories of the 2003 summer when 15,000 people died.
Meanwhile, Dutch and Polish officials have said the heat is damaging crops.
Surgeons, anaesthetists and obstetricians at French private clinics went on strike on Monday, in a row over fees and rising insurance premiums.
Their UCDF union said the strike hit 600 out of some 900 private clinics.
Some clinics have closed, while others are open but without operating theatres.
French officials have expressed fears that public hospitals could be overwhelmed during the heatwave if private facilities are unavailable.
France is on orange alert - the second highest warning level - for about half of the country.
Temperatures well above 30C (86F) have been registered across Europe.
Patrick Pelloux, head of France's Association of Hospital Emergency Ward Doctors, warned that many sick people would have to wait on stretchers for hospital beds, French radio reported.
He voiced particular concern about the Ile-de-France area, covering Paris.
Mr Pelloux called for a law to prevent hospital managers from cutting bed numbers in summer.
The latest victim of the heatwave was a 90-year-old woman found dead in Orly, near Paris. Her body temperature was 41C (105.8F).
The heatwave is expected to intensify on Wednesday, French media report.
Mr Bertrand also urged people to visit elderly friends, neighbours and relatives - especially those living on their own during the holiday season, when the French traditionally vacate the cities for the countryside, including many care workers, doctors and nurses.
The government is issuing warnings on radio and TV, reminding people to drink enough water and stay out of the sun, the BBC's Caroline Wyatt reports from Paris.
Elsewhere in Europe:
- Polish Agriculture Minister Andrzej Lepper warned that Poland's cereal harvest would slump by 20% because of the drought
- Dutch agriculture officials warned that the country's potato crop was likely to be poor
- Spain's oldest nuclear power station - the Santa Maria de Garona plant - was shut down on Sunday because the Ebro river water became too hot to cool its reactor.