Temporary "emergency" electrical power supply improvisation
You cannot rely on procedures to avoid backfeeding

So it is not enough to switch off the main breaker and then tie a string on your finger to remind you not to turn it on.  You must actually remove the meter, or output wires from the house side of the main breaker (many European panels have wires run between main and branch circuit breakers). The panel must be rendered safe from someone turning on the main breaker or any other breaker unawares, and backfeeding must be impossible.

My usual approach is to rewire the panel "permanently", then rewire it back

The hot and neutral from the various circuits can be attached to an inlet or if your Code allows, a short electric cord with a proper strain relief and fittings leaving the panel.  

Internal to the panel, you jumper it around so any circuits which need power are pulling hot and neutral pull from the inlet/cord.  Do all this properly, using permanent wiring methods to Code. 

Then, when power returns, you put it back the way it originally was.  

You must be careful not to "paint yourself into a corner", i.e. don't disconnect anything that will be energized when power returns.  In fact, while working, assume the power company could restore power at any moment.