* Lionel Messi grabbed hat-trick for Barca
* City's former Barca keeper Claudio Bravo was sent off
* Home substitute Mathieu also received red card
* Neymar penalty saved before he notched fourth goal
* Teams to meet again at the Etihad Stadium on Nov. 1 (Adds quotes)
By Richard Martin
BARCELONA, Oct 19 (Reuters) - Lionel Messi scored his second hat-trick in as many Champions League games to give Barcelona a 4-0 win over Manchester City as Pep Guardiola and Claudio Bravo endured harrowing returns to the Nou Camp on Wednesday.
A slip by Fernandinho allowed Messi to round former team mate Bravo and open the scoring in the 17th minute and the keeper completed a miserable night by getting sent off in the second half for handling outside the area after losing the ball.
The magical Messi hammered in his second from the edge of the area after turning two City defenders and pounced on a Luis Suarez lay-off to complete his treble in the 69th minute.
Barca substitute Jeremy Mathieu was then dismissed for two bookable offences. Neymar had a late penalty saved by substitute keeper Willy Caballero but the Brazilian made amends by scoring the fourth goal in the 89th minute.
"It's difficult to play with Barcelona against 11 men. With 10, it was over," said Guardiola, while adamant that he would not blame Bravo for the error which effectively ended his side's night.
"Mistakes happen. He is sad but he will learn a lot. As long as I'm a coach I'll keep playing out from the goalkeeper," the coach added.
Luis Enrique's team lead the group with nine points followed by Guardiola's City on four. Borussia Moenchengladbach have three points while Celtic are bottom with one.
Guardiola, who surprisingly left out key marksman Sergio Aguero to accommodate more midfield strength, watched his old side pressed back in their own half for 15 minutes until Fernandinho's error and Messi's determination changed the course of events.
Messi began the move and fought to win the ball back before threading a pass to Andres Iniesta. Fernandinho tripped over just as he looked set to clear, leaving the Argentine an easy task to score.
City did not wilt, Nolito and Ilkay Guendogan having shots saved by Marc-Andre ter Stegen, while at the other end Suarez was denied by Bravo and also shot wide.
The visitors collapsed after the break, though, their downfall starting when Bravo came out of his area and lost the ball to Suarez before using his hands to stop the Uruguayan's attempted chip.
"Perhaps the result is a bit unfair on them but if you commit grave errors and you have three forwards like ours against you, this is what happens," Barca coach Luis Enrique said.
Messi's second strike was reminiscent of a goal he scored against Guardiola's Bayern Munich in the 2015 semi-finals and Suarez's generosity allowed the Argentine to knock in the third with simplicity.
Caballero thwarted Neymar from the spot but the Brazilian still had time for a neat fourth, heaping more pain on Barca's most decorated coach Guardiola. (Editing by Tony Jimenez and Ian Chadband)