Around 500 people were at the "Fathers' Day" bank holiday party in Dabel, east of Schwerin, Mecklenburg, when the lightning struck shortly after midday. It hit not only a tree but also a nearby electricity junction box.
"It was a really great Fathers' Day party. Then the lightning came," said Andreas Küster, a volunteer fireman who was there.
"I heard a loud bang. People were falling over and I went running over to help the injured.
"There was panic, hysteria and screaming, everyone was trying to get away."
The festival was being held in a field on the edge of the Holzendorfer Lake.
Police in Rostock said many people were hurt after being catapulted through the air by the shock, while the effect was probably amplified when the lightning hit the junction box.
Four of the most badly injured were flown by helicopter to nearby hospitals while others were taken by ambulances and fire trucks.
The German Weather Service (DWD) had warned of 'notable' weather at lunchtime and then counted 25 lightning bolts in ten minutes from the storm cloud which seemed to have been responsible for the strike at Dabel.