A 17-year-old boy attending a friend’s birthday party in Williamsburg, Brooklyn, was fatally stabbed when a brawl broke out during a pickup football game, police said yesterday.

“He was a kid, not even 18,” said Edgar Soto’s crying mother, Juana, outside her Bronx home.

Edgar, his mom and his sister, Giselle, 12, were at the celebration Saturday night when he and some pals headed out to throw the pigskin around in a nearby parking lot, on Grand and Lorimer streets, according to cops.

Just after 11:30 p.m., another group of people approached the kids and the deadly violence broke out, police said.