Police have defended the actions of their officers.

A video of a violent arrest posted on social media shows officers arresting an armed teen who had robbed a pregnant woman, police say.

The video has been circulating on Facebook and was filmed on Sunday at 4pm. Comments on the video condemn police actions.

Inspector Sunny Patel said the man was disguised with a cap and a bandana over his face when he entered the Northcote Bakery on Auckland's North Shore armed with a large glass bottle.

He allegedly demanded money from the heavily pregnant shop worker, Patel said.

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"The shop worker complied with his demands and handed over $50. The offender then left the store. The victim is 33-weeks pregnant and felt threatened and shaken."

Police were called and began searching the area. They found the man, who then ran away down an alley and side streets.

Police chased the man for nearly a kilometre, until they came to an alleyway.

Patel said officers arresting the man saw he still had the bottle in his hand.

"The offender continued to hold the heavy glass bottle. As can be seen in the video, there were children around at the time and the two staff were faced with a large, armed and dangerous offender who had just carried out a robbery on a pregnant woman."

It is at this point in the video that one officer can be seen repeatedly punching a man while another holds him down on the ground.

Patel said the officers managed to disarm the man but he refused to listen to instructions and tried to reach for something in his pocket.

The offender was charged with robbery and would appear on Monday in the North Shore District Court.

Patel said it was a reality of policing that people would "sometimes resist arrest in a violent and dangerous manner and when this happens our officers are required to use force to ensure both the public and our staff are kept safe".