An international manhunt is under way for two men who stole a necklace decorated with more than 600 diamonds from a museum in Vienna after buying a ticket for the exhibition in which it was being displayed.

Police said the two suspects smashed a glass display case at the Vienna Museum of Applied Arts (MAK) on Thursday afternoon and grabbed the platinum and diamond necklace, which is believed to be worth more than $4m (£3m; €3.5m).

The necklace, which was designed for the former Egyptian royal family, was part of an exhibition on the jewellers Van Cleef & Arpels, due to run at MAK until September.

The theft is the latest in a series of spectacular heists of artefacts at European museums.

Vienna police released photos of the two suspects, who were unmasked, captured on security cameras. They are appealing for the public to come forward with information.

The police said in a statement that one of the perpetrators was also believed to have been carrying what was thought to be a real firearm.

After seizing the necklace, they fled "towards the Vienna Stadtpark" just south of the museum, they said.

Lilli Hollein, MAK general director, said "the theft of a highly valuable piece of jewellery… has hit us hard".

She said in a statement that MAK had developed a security plan with Van Cleef & Arpels specifically for the exhibition, adding: "Security issues were taken very seriously and all security measures were carefully prepared."

Hollein said staff had reacted calmly and followed the established procedures when the "armed incident" occurred, and that no one had been injured.

She added that MAK had thoroughly reviewed its security measures ahead of the exhibition, "particularly in light of the incidents at major museums in recent years".

Also on Thursday was one of the most important troves of prehistoric jewellery and artefacts was stolen from a Spanish museum.

Four Renaissance paintings were stolen from a museum in Sicily earlier this month, while three paintings by Renoir, Cézanne and Matisse worth millions of euros were taken from a museum in Parma earlier in the year.

These heists came after the brazen daylight raid on the Louvre in Paris, in which a masked gang stole priceless jewels last October.

The string of thefts have raised questions about the security of European cultural institutions.

Europol said museum thefts were "becoming increasingly aggressive, with a shift towards violent and forceful methods", adding this "may indicate the involvement of new criminal networks".

The necklace stolen in Vienna on Friday was designed for the former Egyptian royal family and was worn by Queen Nazli in 1939 for the marriage of her daughter Fawzia to the future Shah of Iran, Mohammad Reza Pahlavi.

Van Cleef & Arpels describes it as a "triumph of white jewellery". The diamonds on the necklace weigh in excess of 200 carats.

When it was auctioned a decade ago, it had an estimated worth of $3.6m-$4.6m.

Police dog units have been deployed in Vienna and the province of Lower Austria, while forensic experts are analysing evidence and CCTV footage.

The exhibition has now been reopened.