Daring thieves mingled with visitors at an exhibition in the Doge Palace in Venice and stole jewellery on display worth thousands of euro.

At least two thieves are thought to have entered the exhibition of Indian and Indian-inspired gems and artefacts owned by Qatari Sheikh Hamad bin Abdullah al-Thani, mixing with other visitors.

Then, while one kept watch, the other opened the display case and grabbed a jewelled golden brooch and a pair of earrings, among other items.

The thieves, who police said appeared to have been part of a larger gang, seemed to have been able to delay the museum's alarm system which only sounded when they were already escaping.

"We are certainly dealing with very skilled professionals, who got away with it even though the building and the rooms were kitted out with very advanced technological systems," the canal city's police commissioner Vito Gagliardi said.

The jewellery, which included diamonds, gold and platinum, had been assigned a customs value of just 30,000 euros ($36,084), but they were really worth "a few million euros", Gagliardi said.