A goal conceded on either side of half-time had the Gunners facing the prospect of their first defeat at the Emirates. The introduction of Emmanuel Adebayor swung the balance back in Arsenal's favour, as he scored one himself within minutes of his taking to the field and had a hand in Gilberto Silva's equaliser.
Robin Van Persie should have put the Gunners ahead after just three minutes when he met an Emmanuel Eboue cross from the right with a powerful header that was spectacularly saved by David James.
Portsmouth responded almost immediately with a fine passing move that began in their own half and ended with Glen Johnson beating two Gunners defenders, before firing a shot from the edge of the area that was gratefully gathered by Jens Lehmann.
Theo Walcott, who replaced the injured Freddie Ljungberg on seven minutes, put the accelerator down on 19 minutes to breeze past the Pompey defence, but his cross come shot was fired straight into the midriff of a relieved James.
Portsmouth once again responded positively with some good work down the right hand side that culminated in Matthew Taylor testing Lehmann with a fine header from Gary O'Neil's flighted ball from the right.
James was again on hand to deny the Gunners what looked like a certain goal after 24 minutes. The Portsmouth keeper would have been expected to save Cesc Fabregas' 20-yard drive, but the ferocious strike bounced off the former England keeper and fell to the lurking Van Persie.
A goal looked certain as the Dutchman shot low towards the right corner but James guessed right and blocked the ball again, to the frustration of the Gunners.
Portsmouth stepped up the pressure and were rewarded for their endeavour as half-time beckoned. A free-kick floated in by Matthew Taylor was headed against the post by David Thompson and Noe Pamarot followed up to head home the rebound.
Any half-time plans that Arsene Wenger hatched in the dressing room were dented within the first minute of the second half when Taylor doubled the visitors' lead with a half-volley from the left corner of the area that flew past Lehmann.
The Gunners were denied a penalty on 55 minutes when Adebayor looked to be upended by Linvoy Primus, but referee Steve Bennett waived the claims away.
The striker was happier three minutes later when he reduced the arrears. Alex Hleb had found Walcott on the right, he sped past Pamarot to cross a low ball into the box. It eluded Primus and Adebayor was on hand to fire home.
Adebayor also played a part in Arsenal's equaliser. Kolo Toure fired a rocket of a shot from 20 yards out that James did well to parry, but only as far as Adebayor, again on the left.
The striker lifted a cross over to Walcott on the far post, and the youngster scrambled the ball back to Gilberto, who shot across goal and into the far left corner beyond James.
The Gunners came close to taking the lead on 80 minutes when Gilberto broke down the right side and burst into the penalty area, but his shot rolled agonisingly wide of the left post.
Mathieu Flamini, on as a replacement at right-back for the injured Eboue, shaved the right post with a shot in injury time and another penalty claim was waived away by Mr Bennett at the death.