Madonna asked two strangers to pay for a cup of coffee after finding she didn't have enough money.
The singer, her husband Guy Ritchie and children Lourdes and Rocco, stopped at a cafe during a walk in Hyde Park.
Madonna approached two sisters and their friend and asked if they would give her £1.20 (€1.88) so she could pay the bill. Newspaper reports say they gave her £2 (€3.14).
Two of the girls, named Mimi and Titi, said: "It was a very, very strange and unusual experience. For someone with her success, what happened was unbelievable."