When two couples entered the exclusive Harry Winston store in Paris yesterday they appeared to be just like the other wealthy sophisticates who covet fine jewellery.

But the staff were soon to discover something was very, very wrong.

The two women were really men dressed in drag and their partners were carrying guns.

In 15 minutes they pulled off one of the world's biggest jewellery robberies, herding 15 staff into a corner, hitting some over the head, and getting some to fill sacks with diamonds, gold, pearls and cash before making off with a haul worth £70m.

It was the biggest theft in French history and understood to be the second-biggest jewellery heist ever in Europe.

Police said the thieves were "terrifyingly efficient" and had meticulously prepared the robbery on the upmarket Avenue Montaigne - even addressing some of the shop employees by their first names and going straight for the store's secret safe.

French police said that Harry Winston - the self-proclaimed king of diamonds and supplier to monarchs, aristocrats and film stars - had fallen victim to a highly professional and well-informed gang.

No shots were fired in Thursday's hold-up but the robbers hit several staff members with their gun butts and reportedly waved a grenade around.

Yesterday, the boutique on the expensive Avenue Montaigne in central Paris was closed and three of the five window displays were empty.

The gang is reported to have known the location of some jewellery storage cases hidden out of sight of the public, suggesting that they either had inside knowledge or had carried out extensive surveillance before striking.

Police confirmed they were dealing with "major pros".

No leads were ruled out, including the possibility that the robbers may have connections in eastern Europe, where the expensive haul could be sold, unlike in western Europe where such high-value stolen goods would be quickly detected.

Police described the eastern European market as a "new El Dorado for traffickers" trying to get rid of high-end stolen goods.

Members of France's elite Detective Squad scrutinised the premises for clues, studying security camera footage and the alarm mechanism, which is linked to a centre in Switzerland.

Investigators also questioned customers, employees and pedestrians while Harry Winston's head branch in New York said it was working closely with the French authorities.

"We are co-operating with the authorities in their investigation. Our first concern is the well-being of our employees," the company said in a statement.

The store which specialises in ultra-luxury jewellery, was hit by another heist in October last year when armed robbers stole an estimated £8.6m worth of valuables.

After that incident an insurance company offered a £250,000 reward for information leading to the thieves' arrest but they were never found.

The Harry Winston jewellery chain has sold precious stones to royal families and lent them to film stars for events such as the Cannes film festival or the Oscars in Hollywood.

The business was founded by Jacob Winston in 1888 in New York before Harry, his son, transformed it into one of the world's most celebrated jewellery chains with an exclusive clientele.

His trademark quip was: "People will stare. Make it worth their while."

His customers have included the Queen, Diana, Princess of Wales, Sophia Loren, Elizabeth Taylor and Aristotle Onassis.

The company's exclusive status was sealed after its name was sung by Marilyn Monroe in the song Diamonds are a Girls Best Friend, in the 1953 film Gentlemen Prefer Blondes.

"Talk to me, Harry Winston, tell me all about it," she sang.

Several leading actresses have since shown off their Harry Winston diamonds on the red carpet.

Gwyneth Paltrow wore a platinum 40-carat diamond necklace when she collected the Oscar for Best Actress for her part in Shakespeare in Love while Halle Berry, Jennifer Lopez and Hilary Swank have also worn Harry Winston gems.

France's biggest theft until this week occurred in 1992, when robbers got away with cash worth £27m from a Bank of France branch in Toulon.

Belgium holds the record for the world's biggest jewellery heist. Up to £90m in diamonds and cash were taken from more than 120 vaults at a heavily guarded diamond exchange in Antwerp five years ago.

The French jewellery industry has reported 132 armed robberies so far this year, a 20% rise compared with the first 11 months of 2007.

Police sources suggest the rise could be linked to the arrival in the west of eastern European gangs.