The 21-year-old, who has never scored a competitive goal for the club, claimed a first-half hat-trick and added another goal in the second half to upstage star signing Alexis Sanchez, who made his first appearance as a late substitute.

Costa Rica forward Joel Campbell, back at Arsenal after three seasons out on loan, scored the hosts' other goal, with Nicolas Gaitan replying for Europa League runners-up Benfica.

The game at the Emirates Stadium followed the 2-2 draw between Valencia and Monaco in the first game of the four-team friendly competition, which concludes on Sunday.

Arsenal manager Arsene Wenger used the occasion to cast his eye over some fringe players, with new signing Calum Chambers -- formerly of Southampton -- starting in an unfamiliar position at centre-back alongside fellow full-back Nacho Monreal.

After Campbell had seen an early penalty claim waved away, Gaitan gave Arsenal a scare with an adroit half-volley that skimmed the crossbar.

But after Campbell had scuffed wide from Aaron Ramsey's dinked pass and Sanogo had steered a low Hector Bellerin cross the wrong side of the near post, the hosts clicked into gear in devastating fashion.

Sanogo broke the deadlock in the 26th minute with a clever back-heeled finish from Ramsey's right-wing cross.

The adventurous Bellerin then teed up Campbell to bludgeon home a left-foot volley in the 40th minute.

Improbably, there was time for Sanogo to complete his hat-trick before the interval.

He tucked away a volleyed cross from Campbell in the 44th minute and then flicked in a centre from Kieran Gibbs at the second attempt in stoppage time.

The towering former Auxerre striker claimed his fourth four minutes into the second period, rolling the ball into an empty net after Benfica goalkeeper Moraes Artur had parried from Ramsey.

Gaitan headed in from a long throw-in with just under half an hour remaining, but Arsenal's fans were given further reason to cheer when Chile forward Sanchez replaced Campbell to make his debut following his switch from Barcelona for a fee of around �30 million ($50.4 million, 37.6 million euros).

Arsenal's new number 17 almost created a goal for fellow substitute Francis Coquelin in injury time, but the young midfielder's shot came back off the post.

Arsenal play Monaco on Sunday before facing Manchester City in the Community Shield next weekend, while Benfica will close the tournament against Valencia.