Two teenage girls have been arrested after another girl was allegedly assaulted at a beach in Melbourne’s southeast.

Police said a 13-year-old girl was at Mordialloc Beach about 1.30pm on December 17 when she was approached by two 15-year-old girls.

It is alleged the younger girl was assaulted near a toilet block close to Mordialloc Pier, before a man believed to be aged between 50 and 60 intervened.

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Police allege the two girls then fled the scene, leaving the 13-year-old with minor injuries.

Following investigations, police arrested the two girls — both from the Casey area — on Friday and interviewed them.

They were later released pending further inquiries.

Investigators are appealing for witnesses, including anyone with dashcam or CCTV footage from the area at the time.

Police are also keen to speak with the man who intervened in the alleged assault.

The riot squad was called to the same beach on the same day after hundreds of teenagers stormed a Woolworths and became involved in a brawl at Mordialloc Pier.

The AirWing was also called in to monitor the chaos from overhead.

The teenagers even turned on police, throwing bottles and other objects at them.

It is not yet clear if the two incidents are linked.

Hundreds of teens stormed a Woolworths in Mordialloc before rushing out to a nearby beach and brawling. Credit: 7NEWS