A storm front caused damage in the Rhineland on Thursday evening.
Duisburg was particularly affected.
The phones in the Baerl district failed, reported the fire brigade, which had to process 500 emergency calls in a short time.
In the case of operations without danger to people, it could take a while for the fire brigade to arrive, it said.
In neighboring Mülheim an der Ruhr, an approximately 20 meter high oak tree was probably split by a lightning strike.
Half of a tree fell on a two-storey house and damaged the roof.
A car was buried by the tree.
People were not harmed.
In Krefeld, streets and cellars were flooded, trees fell down - including on the train route between Krefeld and Meerbusch, which was temporarily blocked as a result.
In Cologne, the fire brigade reported 32 operations via Twitter.
Trees had fallen there too.
Storms also caused heavy rain and hail in southern Germany on Thursday evening.
In Kempten, a lightning strike shut down the railway signal box, and trains were temporarily stopped on the route to Kaufering.
In Oberstdorf there were mudslides, several roads were blocked by debris and water, and the cellars and stables were full of water.
In the district of Garmisch-Partenkirchen, the fire brigade had to go out on 47 missions on Thursday evening.
A spokesman for the Oberland integrated control center said that hail had destroyed skylights and that water had entered Aldi in Oberau.
The federal highway 2 was briefly impassable in Oberau.
Cellars were flooded.
There were two storm operations in the Bad Tölz district.
The German Weather Service had warned of the thunderstorm front with heavy rain.
Strong thunderstorms could also occur on Friday, especially in the east and north-east, the DWD said on Thursday.
The temperatures there rise to a muggy 25 to 30 degrees.
In the rest of Germany, the values are between 18 and 24 degrees.
In the north-west and south-west, shower-like rainfall is also to be expected, which will move east over the course of the day.
In the western half it will generally remain dry with loosening cloud cover.
In the night to Saturday, the rain and clouds will largely say goodbye in the north-east and on the Alps and thunderstorms are hardly to be expected.
It cools down to 15 to 8 degrees.
According to the meteorologists, the sun will often shine on Saturday.
There are only a few clouds then.
In the north it tends to be more frequent.
However, showers are rarely to be expected.
The temperatures rise to 24 to 28 degrees.
Sunday night will be clear with lows of 16 to 10 degrees.
Thunderstorms and showers are still possible on Sunday in the extreme south-west and in the Alps.
Otherwise it will be sunny again with a hot 26 to 32 degrees and partly cloudy.
Only in the north of Schleswig-Holstein and along the coast will it get a little cooler.