The benchmark from Perez came halfway through the hour-long session and, despite a late effort from Alonso, the Red Bull driver's time stood firm.
In warm 27-degrees C conditions, albeit unrepresentative of those the drivers will face in the night-time qualifying and race sessions, Williams' Alex Albon was first onto the scene as the Grove team sought to get the early running in, with new team-mate Logan Sargeant following him out of the pitlane.
As the two Williams pitted after a single exploratory lap, Lando Norris was afforded the chance to set the first recorded time of the year - a 1m37.462s on the medium tyres.
This was quickly shattered by Perez, who laid down a 1m35.069s - nearly five seconds away from the 1m30.305s he'd recorded in testing on C4 Pirelli tyres, incidentally not a compound that will be in use this weekend.
But Fernando Alonso swung in to displace the Mexican with a 1m35.048s, and while Max Verstappen rifled in his first time a couple of minutes later, the defending champion was 0.381s away.
Alonso held the fort for the opening 10 minutes, but Perez then later eclipsed the Aston Martin with a 1m34.343s as the first quarter of the session begun to elapse.
The gap closed slightly when Alonso set a 1m34.867s, but the early advantage continued to belong to Perez as teams used the early phases of the session to explore set-ups and with the customary daubing of flow-vis paint to inspect the aerodynamics.
After a quiet opening half-hour, Perez was now armed with a set of soft tyres and crushed his own benchmark by nearly two seconds, posting a 1m32.758s to flex his advantage at the top. He was followed across the line by team-mate Verstappen, whose own soft-tyre benchmark was 0.6s off the pace.
Drivers then began to explore the limits of the cars more freely following a tentative opening half-hour, and mistakes began to creep in; Alonso had a wide moment at Turn 4 seconds before Norris went straight on at Turn 1, the Briton bleeding off the brakes before the front tyres had a chance to lock.
Carlos Sainz then suffered a spin at Turn 9 as the rear of his car stepped out through the curved braking zone; but the Spaniard was able to contain it and corral his Ferrari into facing the right way. He later attributed this over the radio to cold tyres on the right-hand side.
Norris moved up to third on soft tyres after the flurry of mishaps were largely contained in the session's third quarter, albeit 1.4s down on Perez's benchmark, and was briefly accosted by Zhou Guanyu before Charles Leclerc split them with fourth place.
Alonso began his own soft-tyre lap in a bid to knock Perez off of his perch, and despite getting within a tenth by the end of the first sector, could only set a 1m33.196s - 0.4s adrift.
This proved to be the only siege that Perez had to withstand, and ended the session as he did testing with the fastest time.
Verstappen was third-fastest, just under two tenths behind Alonso, with an 0.8s advantage over Norris and Leclerc.
Lance Stroll, who returned to action after he missed testing due to a broken wrist, made a slow start to the session as his Aston Martin team sought to ensure the Canadian was suffering no ill effects following an operation.
Stroll driver managed to bag 17 laps by the end, placing sixth with a 1m34.298s and a tenth clear of Haas' Kevin Magnussen.
Zhou had an issue with the positioning of his pedals in the car, but was told the team could not realign them until the break before the second free practice session. The Chinese driver was eighth quickest, ahead of Alfa Romeo squadmate Valtteri Bottas.
Lewis Hamilton completed the top 10, but neither Mercedes driver attempted a lap on soft tyres - George Russell in the other W14 was 11th quickest and just 0.05s off his team-mate.
McLaren's Oscar Piastri was 12th to finish as the top rookie, albeit notably suffering a minor snap of oversteer on the downhill run out of Turn 6/7 during the opening 10 minutes of the session as he continued to adapt to the rigours of F1.
Nyck de Vries was 16th for AlphaTauri, 0.4s off team-mate Yuki Tsunoda, as Sargeant was the last of the three rookies in 18th, having reported that he could "smell something burning" early on in the session. He finished ahead of Albon, as Sainz brought up the rear following his earlier spin.
F1 Bahrain GP - FP1 results
| Cla | Driver | Chassis | Engine | Time | Gap | |
|---|---|---|---|---|---|---|
| 1 |  Sergio Perez |
 |
Red Bull | Red Bull | 1'32.758 | |
| 2 |  Fernando Alonso |
 |
Aston Martin | Mercedes | 1'33.196 | 0.438 |
| 3 |  Max Verstappen |
 |
Red Bull | Red Bull | 1'33.375 | 0.617 |
| 4 |  Lando Norris |
 |
McLaren | Mercedes | 1'34.165 | 1.407 |
| 5 |  Charles Leclerc |
 |
Ferrari | Ferrari | 1'34.257 | 1.499 |
| 6 |  Lance Stroll |
 |
Aston Martin | Mercedes | 1'34.298 | 1.540 |
| 7 |  Kevin Magnussen |
 |
Haas | Ferrari | 1'34.402 | 1.644 |
| 8 |  Zhou Guanyu |
 |
Alfa Romeo | Ferrari | 1'34.575 | 1.817 |
| 9 |  Valtteri Bottas |
 |
Alfa Romeo | Ferrari | 1'34.689 | 1.931 |
| 10 | Lewis Hamilton |
 |
Mercedes | Mercedes | 1'34.917 | 2.159 |
| 11 | George Russell |
 |
Mercedes | Mercedes | 1'34.966 | 2.208 |
| 12 |  Oscar Piastri |
 |
McLaren | Mercedes | 1'34.997 | 2.239 |
| 13 |  Yuki Tsunoda |
 |
AlphaTauri | Red Bull | 1'35.015 | 2.257 |
| 14 |  Nico Hulkenberg |
 |
Haas | Ferrari | 1'35.043 | 2.285 |
| 15 |  Esteban Ocon |
 |
Alpine | Renault | 1'35.105 | 2.347 |
| 16 | Nyck de Vries |
 |
AlphaTauri | Red Bull | 1'35.402 | 2.644 |
| 17 | Pierre Gasly |
 |
Alpine | Renault | 1'35.455 | 2.697 |
| 18 |  Logan Sargeant |
 |
Williams | Mercedes | 1'35.749 | 2.991 |
| 19 |  Alex Albon |
 |
Williams | Mercedes | 1'36.018 | 3.260 |
| 20 | Carlos Sainz |
 |
Ferrari | Ferrari | 1'36.072 | 3.314 |
| View full results | ||||||
