Jonathan Trott was out early for five and captain Alastair Cook (62) fell shortly before lunch in what seemed a hammer blow to the hosts' hopes of salvaging the match after Australia amassed 527 for seven declared.
However, Pietersen (78 not out) and Bell (51 not out) survived with some fortune to frustrate the tourists, who trail 2-0 in the five-match series and must win here at Old Trafford to have a chance of regaining the urn.
On 62, Pietersen came down the pitch and was struck on the pads but Shane Watson's appeal for lbw was turned down.
Australia captain Michael Clarke decided against a review but the team informed him from the balcony with a raised finger that televisions replays showed Pietersen would have been out.
Clarke's shoulders immediately slumped, such was his disappointment at failing to snare England's most explosive batsman.
Pietersen, with a poor record at the Manchester ground and this series, got off to his usual nervous start but gradually settled with two straight pulled boundaries off Mitchell Starc.
He then launched off-spinner Nathan Lyon for two successive sixes down the ground to bring up his half-century but Australia did well to otherwise keep the run rate down.
The slow pace of play led to the almost obligatory Mexican wave through the capacity crowd and three beach balls found their way on to the field.
By tea, Pietersen was seeing the second new ball like the proverbial beach ball but was partly limiting his attacking instincts with England still in a pickle.
Bell, who hit tons in the first two tests at Trent Bridge and Lord's as England hone in on a third straight Ashes series win, was given not out on four after a Starc appeal for caught behind. Television technology suggested he may have edged.
The elegant right-hander continued unabashed but like Pietersen combined some delightful shots with steadfast defence as they brought up their 100 partnership at the break.
Trott, who came in at four after nightwatchman Tim Bresnan followed opener Joe Root back to the pavilion late on day two, continued his patchy series by awkwardly edging fast bowler Ryan Harris to Clarke in the slips early in the day.
Cook, who destroyed Australia with his big scores in the last tour Down Under, looked well set but went when he nicked left-arm quick Starc down the leg side and wicketkeeper Brad Haddin took a smart diving catch.
Forecast showers for later on Saturday may help England's cause but the first two sessions were played under wispy cloud.
(Editing by Pritha Sarkar and Alison Wildey)
