Three children were fighting for their lives after a sudden storm sent a bolt of lightning crashing down on a birthday party in a Paris park.
A spokesman for Paris' fire service credited an off-duty firefighter with playing a vital role in getting medical help to the victims.
Another group of people at a children's football match in Germany was also hit by lightning on Saturday, leaving three adults seriously injured.
The birthday group in Paris had sought shelter under a tree at Parc Monceau, an area popular with families, when a lightning bolt hit, according to Paris fire service spokesman Eric Moulin.
He said Pascal Gremillet, an off-duty firefighter, was visiting a museum nearby when he noticed the commotion and found nine of the 11 victims lying unconscious. He immediately went to work.
"He saw who was the most seriously injured. He did a quick triage of the victims. He did first aid. He alerted the rescue services," Moulin said. "Without his actions, it would have been much worse."
Moulin said six of those hit were seriously injured, with three children and one adult fighting for their lives. He said five others had been slightly injured by the lightning, including four children.
Footage shot by the fire service showed a dramatic scene at a nearby bank that was commandeered as a makeshift treatment centre, with children wrapped in gold thermal blankets sitting and lying on the building's tiled floor as firefighters administered first aid before evacuating the victims to hospitals.
Two small feet, smudged with what looked like soot, stuck out from underneath one of the blankets. Moulin gave the children's ages as around nine.
An Associated Press reporter at the scene saw an abandoned pair of children's glasses and jacket near the tree. White-and-red tape was strung around the area and whistle-blowing park wardens ushered weekend joggers out of the park, which was swiftly closed.
"This accident is extremely rare in the Paris region," Professor Pierre Carli, an emergency medical services official, told reporters.
One Paris resident who lives near the park and saw the lightning strike said it was rare to see such a wild storm hitting the French capital.
"It was dramatic," said Jean-Louis Laurens.
Emergency doctor Pierre Carli said one child in Paris is on life support.
He said three other seriously injured children are out of immediate danger and are under observation. Four other children who were lightly wounded by the lightning strike are "simply under observation".
He said two men and one woman at the party were also injured, and the woman was the mother of some of the injured children.
Storms had violent consequences elsewhere in Europe. Police in Germany said 35 people were taken to hospital after lightning struck a children's football match. Three adults were seriously injured in the strike in Hoppstadten, including the referee.
Another 32 people, among them 30 children ranging in age from nine to 11, were taken to hospital as a precaution, the dpa news agency reported.
The game had just finished when lightning struck suddenly at about 2pm, police spokesman Dominik Lentz told n-tv television.
"According to what everyone present says, there were no clouds in the sky, so this incident couldn't have been expected," he said.