THEY call themselves MBM - the Muslim Brotherhood Movement - a gang of 600 men who boast they are the toughest and best young street fighters of Middle Eastern descent in Sydney.

MBM claims to be the biggest of four new gangs to emerge on Sydney streets in the past year. Its numbers rival those of the state's largest bikie gang, the Rebels.

The sudden appearance of MBM, with its growing membership recruited predominantly from the city's south-western suburbs, has alarmed senior police already battling to combat open warfare among outlaw motorbike gangs.

Even hardened private security guards have expressed concern to police about the indiscriminate "punch and run" tactics of MBM members who, in the past two weeks, have arrived in large numbers at city nightclub venues and who walk the streets in intimidating mobs. But the objectives of MBM - its emblem features two crossed pistols and a hand grenade - and its leadership remain unclear to officers of both the Organised Crime and Gang Squad and Middle Eastern Organised Crime Squad.

Police say that a fortnight ago MBM members embarked upon a campaign of random assaults on men who crossed the path of a mob of about 100 toughs stalking Darlinghurst and Kings Cross during the Gay and Lesbian Mardi Gras.