The Italian set a 2m00.295s on his final attempt to top the timesheet, with Marini just 0.036s behind him.
Bezzecchi, who sits third in the championship standings after the summer break, set an early benchmark of 2m04.821s to lead early on, improving to a 2m00.885s on his fifth lap.
That lap remained quickest for much of the session, before Marini went faster with five minutes left, just five-thousandths of a second better than Bezzecchi.
But Bezzecchi went faster still on his final flying lap, topping the timesheet as he hopes to continue his strong form, having taken a sprint race win and second place in the grand prix at Assen last time out.
The VR46 Ducati pair led from the Pramac duo of Jorge Martin and Johann Zarco, with just two-tenths separating the top four.
With FP1 no longer counting towards Q1 and Q2, it was a more sedate start to the session than usual.
Bezzecchi set an early benchmark before it was bettered by Martin, with a 2m04.506s going quickest as Aprilia’s Maverick Vinales slotted into third.
KTM factory rider Jack Miller went top next, with a 2m02.808s, 0.09s quicker than Bezzecchi in second.
Bezzecchi then improved once more to take the lead, with Miller in second and Vinales in third, while Franco Morbidelli, who will leave Yamaha at the end of the season as Alex Rins replaces him, went fourth.
Aleix Espargaro continued Aprilia’s positive showing, going second quickest next, 0.589s off Bezzecchi’s time, with Miller in third.
The cool conditions at Silverstone proved challenging as the session continued, with Miller sliding at Luffield as riders tackled strong winds.
Bezzecchi led from Espargaro and Marini at the halfway mark, with Gresini Ducati rider Alex Marquez and KTM’s Brad Binder rounding off the top five.
Bezzecchi ran wide at Brooklands into the heavy braking area as conditions continued to prove to be difficult.
With 20 minutes remaining, Morbidelli became the weekend’s first faller after a high side at Luffield, the rear of his Yamaha stepping out and flinging him off, though he walked away unscathed.
Martin improved to third with 10 minutes remaining, with Marini then taking the top spot with a 2m00.831s with five minutes left on the clock.
Espargaro briefly captured the top time, 0.154s faster than Marini, before Zarco went faster still with a 2m00.467s. Marini improved to a 2m00.331s before Bezzecchi finally finished the session top.
Championship leader Francesco Bagnaia appeared frustrated in the closing stages, having had a quiet FP1 which saw him finish 14th.
Finishing his quickest effort, his 14th of 15, the Ducati factory rider shook his head as he crossed the line 12th fastest before being demoted two places.
Miller finished the session fifth, with Espargaro in sixth ahead of Morbidelli. RNF Aprilia’s Raul Fernandez was eighth, with Vinales and Yamaha rider Fabio Quartararo rounding off the top 10.
Marc Marquez, whose future at Honda remains the source of much discussion, was 15th, with a 2m01.586s, 1.2s off the pace.
Pol Espargaro, returning for his first grand prix since March’s season-opening Portuguese Grand Prix, finished the session in 22nd as he got to grips with his GasGas KTM machine.
Several riders had incidents after their practice starts, with Vinales stalling before Ducati rider Enea Bastianini crashed at Becketts on his way to his second practice start, landing on his right shoulder, which he injured in the Portuguese GP sprint race in April.
Iker Lecuona, standing in for Rins at LCR Honda, then stopped on his way back to the pits and was forced to push his bike back.
MotoGP British GP - FP1 results
