Local police chief Salvador Iglesias of the San Juan de Lurigancho district of Lima says police are investigating whether extortionists were behind Tuesday night's attack.
Firefighters say most of the injured had shrapnel wounds in their legs, while four suffered hearing problems and a circus worker was hospitalized with a stomach wound.
Police say they are trying to determine whether the grenade was thrown or planted previously.
Extortion has been on the rise in Peru in recent years, with construction projects, private schools and taxi drivers among frequent targets. Peru's chief prosecutor reported 1,667 complaints of extortion in the first four months of this year.