Experts said the jewels are worth around several million euros in total, but will be very hard to sell on the black market because they are so well known.
Investigators therefore believe the gems may be removed from the jewellery and sold separately.
Venice Police Chief Vito Gagliardi said experts from Rome had immediately been called to the scene.
“It is crucial to understand what didn’t work properly in the security systems,” he said.
“The glass case was opened up as it if were a tin can, while the alarm, if it worked at all, went off late.”
According to initial reports and CCTV footage, two people broke into the case, one concealing the other as they opened it.
The man who broke open the case pocketed a gold brooch and a pair of earrings worth millions of euros.
None of the major pieces on show were stolen.
The alarm went off, but the thieves were able to make a getaway by blending into the large crowd.
Photos of the stolen items have already been sent to London, where their owners are located, so that they can be identified and priced.
The exhibition made Venice the first city in Italy to host the famous Indian gems and jewels from the Al Thani Collection.
Showcasing over 270 pieces, it explored five centuries of the jewelled arts made in and inspired by the Indian subcontinent.
The collection was assembled by Sheikh Hamad bin Abdullah Al Thani, a member of the Qatari royal family.