Larsson struck three minutes into injury-time to clinch a 3-2 triumph at White Hart Lane that halted the Blues' slide down the Barclays Premier League.
After Spurs had been reduced to 10 men following the dismissal of Robbie Keane, former Arsenal midfielder Larsson unleashed an unstoppable 25-yard shot.
McLeish said: "That was a fantastic goal, a dream goal. Tottenham were down to 10 men and a wee bit of space opened up for Sebastian.
"Sebastian had a clear sight and he took the chance. It was a phenomenal winner."
The victory was Birmingham's first at White Hart Lance since 1984 and Blues fans were left biting their nails until the final whistle.
Gary McSheffrey fired the visitors ahead from the spot only for Keane to respond in kind before the Republic of Ireland striker added his second.
A brilliant finish from Cameron Jerome put Birmingham level in the 62nd minute and Larsson pounced three minutes into injury time to end a thrilling contest.
McLeish said: "This was more than I expected. I hoped we would keep Spurs quiet and escape with a dream 1-0 victory where everything went perfect.
"To win 3-2 was certainly not how I thought it would finish. But this is only the start.
"It's a terrific boost for us in the fight to climb up the Premiership, especially as Spurs has such phenomenal quality on show.
"At 2-1 down we needed to get back into the game and we showed the character to do that."
McLeish was non-committal on the three major refereeing decisions from official Phil Dowd - the two penalties and red card - although he hinted that Keane's dismissal was harsh.
The former Scotland boss said: "It's hard to say with the penalties.
"I think ours was deserved and (Spurs boss) Juande Ramos will think theirs was deserved. We probably both have our club's glasses on.
"On Keane I don't want to say because if I call it in his favour then it looks like I'm criticising the referee."
Keane was given his marching orders for a one-footed lunge on Fabrice Muamba in the 68th minute and Ramos felt the offence warranted only a booking.
Spurs announced they are to appeal the red card and Ramos said: "We can see from TV replays that the referee's decision were not right.
"It was not a penalty against Younes Kaboul and Keane's sending-off was harsh. Usually Keane's tackle would get a yellow card.
"The referee was very close to the incident but it was completely unfair. It was a yellow card.
"I'd prefer not to express my opinion of the referee. He has a complicated job but I was not happy so I will not say anything.
"But we will not blame the referee for losing this game. It was Tottenham that lost the game."