The hosts made a breathtaking start, scoring after under 60 seconds as Martinelli ended a six game goal drought by firing home from Martin Odegaard’s pass.
It took the Reds some time to respond in a generally cagey first half, but they eventually forced Aaron Ramsdale into a save through Nunez’s shot.
The striker would not be thwarted a second time though, netting his first club goal in two months as he slid in to convert Luis Diaz’s cross.
The first half looked to be fizzling out from then on, but Arsenal re-took the lead in the fifth minute of injury time as Saka finished from close range following good work by Martinelli.
That gave Mikel Arteta’s side a huge lift going into the break, and Odegaard should have doubled the lead after the resumption when shooting straight at Alisson from close range.
He was made to pay for that miss when Firmino equalised with a pinpoint finish into the bottom far corner minutes later.
That was a rare break forward for Liverpool though, who were constantly on the back-foot. Alisson soon made a terrific double stop from Saka and Gabriel Jesus, seconds before the latter was brought down in the box by Thiago Alcantara.
Referee Michael Oliver immediately awarded a penalty, duly converted by Saka.
Liverpool, who grew more and more frustrated, had no response to going behind for a third time, throwing numerous long balls forward but failing to test Ramsdale, whereas Arsenal had appeals for a further spot kick turned ignored when Jesus went down in the box.
The Reds have now gone without a win in five on the road, while also slipping to 14 points behind Arsenal as their struggles continued.
Flashscore Man of the Match: Gabriel Martinelli (Arsenal)
