A witness is still in shock by the words of a youngster who pleaded with bystanders pinning him down after a jewellery heist on Auckland's Queen St that he was "only 13" and to "let him go".

The robbery, which took place on the city's main shopping strip on Saturday, saw a group of six, welding weapons, smash their way into The Hour Glass store, stealing two bags of jewels.

Members of the public and an off-duty officer jumped in to stop the brazen daylight raid, recovering the gems, and holding on to one person at the scene.

Members of the public detained one of the people involved in a daylight robbery in Auckland's Queen St on Saturday.

A pedestrian, who saw the whole incident unfold, told the Herald, it was an "absolutely terrifying experience".

"After the incident, I asked the caught offender on the ground, 'what the heck do you think you were doing? He replied, 'I'm only 13, let me go'.

"This statement shocked me as the other six boys appeared to be no older than 16. And all had weapons - either crowbars, hammers, hatchets or blades."

The man said members of the public tried to stop the young people but none of them looked afraid.

"They even lashed out at the public, swinging weapons."

The offenders then jumped into a blue getaway car with the boot open, he said.

"One of them was in the boot flailing a crowbar at anyone chasing."

An off-duty police officer was taken to hospital after he was injured while intervening in the robbery.

Another eyewitness, who also did not want to be named, said the off-duty officer was struck with a "sickening blow full on the top of the head".

"All the offenders, without exception, I believe, were armed with a hammer," he said.

"When members of the public started to hesitantly intervene, some [two or three] offenders disengaged from the robbery and started aiming hammer blows at members of the public."

The eyewitness said he was shocked by the amount of sheer violence these young men were prepared to use towards the public.

Yesterday a police spokesperson said the officer was hit in the head by one of the offenders and taken to hospital with moderate injuries.

"The jeweller is still working to determine if all items stolen have been accounted for," said the spokesperson.

Police staff at the scene of a Queen Street jewellery store where young men with baseball bats smashed windows and stole items. Photo / Supplied

One person was apprehended at the scene and inquiries were ongoing to locate the outstanding offenders.

There were about 50-60 people gathered outside the shop when police arrived a short time later.

Anyone with information on the offence was asked to contact police via 105 and quote job number P051476660

Information can also be provided anonymously through Crime Stoppers on 0800 555 111 or www.crimestoppers-nz.org.