Larizia, which stocks items by top designers including Stella McCartney, Alexander McQueen and Dolce & Gabbana, has been targeted 11 times in the past three years.
Manager Marian Zandi was with two of her staff when the men came in.
She said: "They had a knife and threatened us. It took them about seven seconds to steal 12 handbags worth £500 and £800 each. We were very frightened, it was like the movies. We have two shutters and concrete posts outside, but what else can we do?"
One concerned trader said police were "powerless" to stop organised gangs. The man, who was too scared to be identified, said: "The criminals know the police response time is 12 minutes, so they only take a few minutes in the shop.
"They used to carry out raids at night, but because of the added security measures they now come during the day when the shops are open."
A Genevieve spokesman said the company was hoping to install bollards to deter ram-raiders.
He said: "We had two substantial break-ins in April and have spent a fortune improving security.
"We now can't have a window display in the evenings and we have a private security guard overnight. We are an internationally renowned shop giving credibility to the area and thieves know there are wealthy Jewish businesses here.
"The authorities are in self-denial and the police are overwhelmed."
In May, thousands of pounds' worth of sunglasses were stolen from Eyelevel Optometrists in Temple Fortune in the same week as another raid at Larizia.
Eyelevel owner Juliet Most said: "I heard screaming and shouting and then heard the car speed away.
"We live in one of London's finest areas and we need either plain clothed police officers or CCTV cameras in the street."
Last Friday, Barnet Council rubber-stamped proposals to install five CCTV cameras in Temple Fortune, but not until 2011. A spokesman said that date could be moved forward if "crime patterns" changed.
The shopping area is currently not covered by on-street cameras.
Barnet police are investigating Monday's incident and are believed to be studying security footage. The men were all white and in their late 20s. They escaped in a gold coloured car.
Supt David Tucker said: "We take all allegations of crime extremely seriously, particularly from people who are victims more than once."
The rising number of attacks has prompted traders to hire private security staff and claim they will be forced out of business if more is not done to tackle the problem.
