Guardiola led Barca to 14 trophies in a glorious four-year reign between 2008 and 2012, but his homecoming was undone by a series of shocking defensive errors and Messi’s clinical touch on his return from injury.
Messi pounced on Fernandinho’s slip to give Barca a half-time lead before goalkeeper Bravo was sent off for handling Luis Suarez’s shot outside the area.
Two more sumptuous Messi finishes soon followed and Neymar made amends for missing a penalty with a brilliant individual run and finish for Barca’s fourth after Jeremy Mathieu’s red card left both sides with 10 men.
Pique has earlier warned man city that a half-fit Messi could ruin your champions league dreams. City now trail Barca by five points at the top of Group C.
Guardiola’s decision to jettison England number one Joe Hart to bring in Bravo at the start of the season will once again come under the spotlight as the Chilean’s difficult start to life at City cost the visitors dearly.
However, early on it looked as if Guardiola was to get one over on his former teammate, Barca coach Luis Enrique, as City hemmed the hosts inside their own half and enjoyed the vast majority of possession.
The course of the game changed with one huge error on 17 minutes, though, as Fernandinho’s untimely slip left Messi free inside the area and he calmly rounded Bravo to slot home on his first start for a month after a groin injury.
Barca’s injury problems continued as Jordi Alba and Gerard Pique limped off before the break due to hamstring and ankle problems respectively.