Substitute Soren Andersen's late equaliser for City gave the right result to a thriller.
The midday kick-off provided cracking fare for a 13,000 crowd, with both teams committed to attack. After Pompey had made a bright a start, City took an 11th-minute lead when right back Scott Murray exchanged passes with Colin Cramb before shooting low past Aaron Flahavan.
But Portsmouth were level seven minutes later, as David Hillier picked out Sammy Igoe's run behind the home defence and the youngster rounded Keith Welch to net with confidence.
It looked as though the visitors would take the points through a Steve Claridge glancing header on 70 minutes. The experienced striker out-jumped Carl Hutchings to convert Jeff Peron's cross.
But City are not the early season pushovers they were and battled back for Andersen to burst onto a long ball down the left before firing past the advancing Flahavan.
There was just five minutes left, and either side could have snatched victory, as neither was prepared to settle for a point.
Andersen's goal redeemed an earlier miss as he gave a sharp display in the second half as substitute for the injured Cramb.
The Danish striker volleyed badly wide from a Brian Tinnion cross on 59 minutes and two minutes later was booked for a lunging foul on Adrian Whitbread, who continued after lengthy treatment.
The Pompey skipper was a giant at the heart of their busy defence, as city top scorer Ade Akinbiyi threatened on numerous occasions.
City also had a key man at the back in the composed Louis Carey, but it was a forward-dominated match.
Portsmouth's Thomas Thogersen headed off the line from Julian Watts in the first half, while Akinbiyi headed over when unmarked at a Tinnion corner.
At the other end the influential Hillier shot just over from long range, while Igoe was always a bundle of trouble for City's back line.
Unusually, City picked up five cautions, although Andersen's scything foul was the one really bad challenge of a memorable contest.