TWO men believed to be Hells Angels bikies have been arrested after Sydney police stopped a taxi and found a semi-automatic assault rifle and ammunition.

Police say the men aged 23 and 24 were arrested by gangs squad detectives who stopped the cab in Camperdown on Monday night.

They allegedly found a Chinese SKS semi-automatic assault rifle, 40 bullets and two loaded 30-round magazines in the boot.

The men have been charged with possessing a prohibited firearm and remain in custody pending an appearance at Newtown Local Court on Tuesday.