WATCH | Gripping moment knife-wielding suspect is apprehended in Sydney

Three Britons have described the "very surreal" moment they apprehended a knife-wielding man in Sydney moments after he allegedly stabbed a woman to death.

Police and witnesses said the knife-wielding man yelled "Allahu akbar", or "God is great", and attempted to stab several people in the Australian city before being arrested.

Video footage showed a man with a knife on the roof of a car after the attack.

The car drives away and a passer-by confronts the attacker with a fold-up chair.

"Just before the intersection with York Street, a man came running around the corner carrying a knife, and being chased by five or so men," a witness told the Australian Associated Press, adding that he initially thought it was a TV crime show being filmed.

Bystanders restrained the man with chairs and a plastic crate.

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