Watch: Messi scores hat trick for Barcelona vs. Manchester City in Champions League
Pep Guardiola's return to Barcelona did not prove to be a happy homecoming.
Lionel Messi scored a hat trick, Neymar added a fourth and Manchester City finished with its backup goalkeeper in net after Claudio Bravo's mishap got him sent off in the 4-0 Champions League result at a raucous Camp Nou.
Messi struck first to open the scoring in the 17th minute, taking advantage of a slip from Fernandinho after combining with Andres Iniesta in the final third and failing to be picked up as a runner into the box. The sequence started after a turnover from Aleksandar Kolarov.
While Barcelona controlled most of the first half, Manchester City called Marc Andre ter Stegen into action through Ilkay Gundogan. Barcelona was also forced into two defensive substitutions because of injury, with Jordi Alba and Gerard Pique making way for Lucas Digne and Jeremy Mathieu before halftime.
Barcelona was gifted a man advantage when former Barça goalkeeper Claudio Bravo was caught well out of the area and had no choice but to leap and swat Luis Suarez's attempt, which would have been goal-bound.
The hosts seized control with the extra man, as Messi scored his second, lacing a perfectly placed low effort by Bravo's backup, Willy Caballero, to make it 2-0 just after the hour mark.
Messi completed the hat trick in the 69th minute, one-timing home after a wholly unselfish play from Luis Suarez, who could have gone for goal but instead crossed for his teammate in a 2-v-1 situation.
Mathieu received a pair of yellow cards in quick succession to even the field at 10 vs. 10, but by that point the damage had been done, with Barcelona leading 3-0.
Neymar had a chance to make it 4-0 after Messi draw a penalty, but Caballero expertly saved his penalty, for which the Brazilian used a lengthy run-up and a stutter-step before striking it to the left.
He made up for it by toying with John Stones before beating Caballero from close range, capping the 4-0 victory.
Barcelona leads the group with a maximum of nine points from its three games.