James Milner's sending off midway through the first half had appeared to hand the advantage to the Baggies and they took the lead through Shane Long's close-range strike after the break.
However, the response from Roberto Mancini's side was befitting of their status as champions as Dzeko levelled with an 80th-minute header before stunning the Baggies, who had won their previous five home league matches, by clinically finishing off a swift counter-attack in stoppage-time.
While it was harsh on Steve Clarke's team, it was another demonstration from City of their resilience in the face of adversity as they maintained their unbeaten league start to remain four points behind leaders Chelsea.
City threatened for the first time after 15 minutes when Ben Foster was forced to tip a Yaya Toure header over his crossbar from Samir Nasri's corner.
The game soon descended into a scrappy affair, however, with referee Mark Clattenburg required to have words with Toure and Liam Ridgewell after their over-zealous jostling for a corner, while Mario Balotelli was booked for a late barge on Claudio Yacob.
Worse was to follow for the champions in the 23rd minute, though, when Milner was red carded for a professional foul on Long.
Vincent Kompany gifted possession away cheaply around the centre circle, James Morrison's probing pass into space gave Long the chance to burst clean through and Milner sent the Republic of Ireland striker tumbling with a desperate last-ditch lunge.
In what had already been a bad tempered contest, tensions grew further still with Long booked for a handball, while Joe Hart required treatment after being accidentally caught by Graham Dorrans as the Scotland international attempted to latch onto Gareth McAuley's flick.
Mancini's teamtalk appeared to have some impact on City as they emerged revitalised for the second half and within two minutes it required an outstanding save from Baggies goalkeeper Foster to turn Carlos Tevez's fiercely-struck shot away for a corner.
However, City only had themselves to blame when they failed to score on the hour mark after Tevez's free-kick was headed back across goal by Richards only for both Balotelli and Toure to waste close-range chances, the latter blazing over the crossbar.
It proved costly as within seven minutes the Baggies were ahead thanks to Long's fifth club goal of the campaign.
Within four minutes of coming on as a substitute, Peter Odemwingie made an impact as Long broke the offside trap to redirect the Nigerian's low shot into the opposite corner beyond the wrong-footed Hart.
But the home's side's lead was to last just 13 minutes and this time it was Mancini who made an inspired substitution.
On the field just a minute, Dzeko rose highest to glance home Tevez's free-kick off the back of his head.
It was absolutely intentional, yet Foster will have been disappointed with his contribution, having come off his line only to fail to make the punch.
Chances came and went at either end, with Romelu Lukaku unfortunate with a superb bicycle kick.
But the Baggies were ruthlessly punished in added time when another substitute, Sergio Aguero, led a devastating City counter down the left and crossed to the unmarked Dzeko, who kept his cool to fire past Foster.
Source: AFP
Source: AFP