Pompey scored five goals in the last 11 minutes to secure an unlikely point at the Wellington Sports Ground.
Jeff Schlupp and George Taft gave Leicester a two-goal lead before Sam Magri diverted the ball into his own net.
And the visitors seemed to be cruising to victory when Schlupp added his second after the interval, before Nathan Hicks scored the Foxes’ fifth from the penalty spot.
Carl Walshe’s goal on 79 minutes looked to be no more than a consolation – but the Blues kept on scoring.
George Colson and substitute Dan Butler both netted in the space of a minute to give Paul Smalley’s side hope.
Sub Ashley Harris added another and Alex Grant (pictured) scored deep into stoppage time to complete an incredible comeback.
Smalley said: “We were up against one of the most successful under-18 sides of the last three seasons. They were powerful and aggressive with a very direct way of playing.
“Leicester were a constant threat from long throws and set-plays and we were finding it difficult to deal with their aerial attacks.
“But we had been creating great chances ourselves and it just seemed like it wasn’t going to be our day – but what a comeback.
“Carl Walshe had been excellent throughout and deserved his goal. Then, through belief and sheer determination, we found ourselves only 5-4 behind, before equalising at the death.
“We’ll take confidence from that and it will certainly be a motivating factor over the coming weeks.”
Pompey: Gledhill; Tallack, Ryan, Magri, Grant; Seeley (Jennings 63), Stockford (c), Tsovolos (Harris 55), Colson, Martin (Butler 55); Walshe
Sub not used: Fitzharris
Leicester: Barrett; Seysay (Jones 63), Moore (c), Knight, Taft; Tabbutt, Hicks, Robinson (Hopper 52), Norburn; Schlupp, Chamberlain (Brown 65)
Subs not used: Brennan, Paratore
Pompey's under-16 side were beaten 3-1 at home by the same opposition.