Barca won 1-0 in the Nou Camp to complete a 3-1 aggregate win over City, but the scoreline would have been much greater but for a string of saves from Hart.
"He was phenomenal, he stopped everything," Messi said on the Barca website
"We have to congratulate him on a great game because he's a fantastic goalkeeper."
Andres Iniesta added: "Hart was key to keeping them in it and was great in one-on-one situations
Outstanding."
Coach Luis Enrique was happy to see his side progress to the quarter-finals of the Champions League, but felt the scoreline did not reflect their dominance - with Hart the main reason
"The score could have been a rout," he said
"Hart was incredible but we weren't really on towards the end
"We had a lot of chances, which is not normal in this type of game
We deserved to win by a bigger goal difference."
Messi, who turned on the style as he nutmegged both Fernandinho and James Milner during a scintillating display, set up Barca's goal as his cross picked out the unmarked Ivan Rakitic in the 31st minute
Sergio Aguero had a chance to make a game of it late on, but saw his 78th minute penalty saved by Marc-Andre ter Stegen
"Mascherano and Rafinha both told me to which side Aguero was going to shoot," the German goalkeeper said.
"One said left, the other said right
In the end, I just guessed, but I'm happy they tried to help."
Barca will now turn their attention to El Clasico as the Spanish league leaders prepare to take on second-placed Real Madrid at the Nou Camp on Sunday evening
"We're back at a high level and we have to keep it going as long as possible." Messi added as he looked ahead to the match
"We have a lot of respect for this game
They're never in bad shape; they're still dangerous, with impressive individual players and we need to respect more than ever ..
They are very strong and we have to play our game and, with the help of our fans, try to overcome them."
Source : PA
Source: PA