Arsenal were on track for victory after man of the match Jack Wilshere inspired a second-half fightback, firing a wonderful equaliser before teeing up Alexis Sanchez's sublime volley.
But Demichelis rescued a 2-2 draw from a pulsating match when he nodded Aleksandar Kolarov's corner past Wojciech Szczesny in the 84th minute.
It was a bitter blow to Arsenal, who appeared to have done enough to mark Danny Welbeck's debut with a precious victory over their title rivals.
Welbeck scored both goals in England's 2-0 win over Switzerland on Monday but failed to finish a one-on-one with Joe Hart as Arsenal sought reward for a dominant opening.
Instead, his attempted lob of Hart struck the left post in a miss that Manchester United boss Louis van Gael would use as justification for his £16million deadline day sale to Arsenal.
The outstanding forward on display was Sergio Aguero, whose wonderful finish of Jesus Navas' sublime pass opened the scoring for a City side that was deadly on the counter.
A further setback to Arsenal saw full-back Mathieu Debuchy require extended treatment after falling awkwardly before being carried off on a stretcher.
Frank Lampard made his City debut after Yaya Toure played for the Ivory Coast in midweek but he lasted only until half-time, with a foul on Aaron Ramsey that earned a yellow card his headline moment.
Ramsey started in Arsenal's midfield after recovering from an ankle injury, but all eyes were on Welbeck's first appearance for the Gunners.
His first touch was greeted with a roar of approval from home fans and expectation continued to grip the Emirates Stadium as Sanchez tested Hart from long range.
Somehow Aguero threaded a path between three defenders, but the angle was too tight to produce a meaningful shot and Arsenal immediately renewed their offensive.
Welbeck failed to connect with Mesut Ozil's delicate pass and moments later the England forward had a glorious chance to put Arsenal ahead after intercepting David Silva's ill-judged back pass
Only Hart stood between Welbeck and a debut goal but his attempted lob drifted on to the left post
Arsenal had bristled with intent in a superb start, but City's ability to threaten on the counter was highlighted when Navas broke clear and forced a key clearance from Per Mertesacker.
Moments after Sanchez had almost ghosted through City's defence, the champions struck a clinical opening goal after a 50-50 clearance in midfield looped over to an unmarked Navas on the right wing.
The Spaniard set off and delayed his pass perfectly for the arriving Aguero to slide his shot past Szczesny.
Szczesny then had to prod clear a shot by Silva after Pablo Zabaleta exploited Arsenal's left flank - and particularly outclassed full-back Nacho Monreal - once again.
Aguero's goal had clearly deflated the Gunners and it was not until the opening exchanges of the second half that they threatened again - only for Zabaleta to prevent Welbeck from reaching Sanchez's cross.
Just as Arsenal appeared to be fading once more, they struck a 64th-minute equaliser through Wilshere.
Aguero lost the ball in midfield and Sanchez pounced, threading a pass to Ramsey who found Wilshere.
The England star still had work to do, but he expertly rounded Gael Clichy with a wonderful touch before lofting the ball over Hart.
And nine minutes later Wilshere was involved as Arsenal surged ahead through Sanchez, who volleyed his team-mate's header into the net after a failed clearance from Vincent Kompany.
Demichelis capitalised on a mix-up between Laurent Koscielny and Szczesny to plunder City's equaliser and Arsenal were relieved to see the flag to go up after their former player Samir Nasri stabbed the ball into the net in injury time.
Source : PA