Two items of jewellery have been stolen from the Marilyn Monroe exhibition which opened last week at County Hall's Dali Universe.
A gold ring with an M motif, encrusted with diamonds, and a gold bangle, worth £40,000 in total, were snatched from County Hall on the South Bank.
The items were stolen at 3.20pm on Monday when about 150 visitors were in the gallery.
A man who was arrested nearby in connection with the thefts has been bailed to return to a south London police station and police are continuing to look for another man, described as black, slim, 5ft 9in tall and wearing a dark coat over a suit.
Det Insp Tim Forber of Kennington CID, who is leading the investigation, appealed for witnesses and for the return of the jewellery. "It will be very difficult to sell these items," he said.
Marilyn Monroe – Life Of A Legend features more than 250 items, including previously unseen photographs, mementoes from the Hollywood star's childhood and designer dresses.
The gallery's Antonia Spanos said: "The thieves wandered in quite aggressively without buying tickets so our staff were alerted.
"It appears that they pushed one of the cabinets, grabbed what they could and ran.
"Two members of security staff ran after them. They split up and so did our security guards."
One man was eventually apprehended by a member of staff.
"Security guards flagged down police who were in the area. They arrested one of the men who had been caught by our head of security.
"Nobody was hurt and all the security procedures we had in place were effective.
She added: "Some of the artworks in the show were a lot more valuable. But I guess the thieves came in and saw something sparkly and beautiful and grabbed it.
"They were obviously not art lovers because they didn't know what to head for."
The jewellery, which was on loan from auction house Cooper Owen, was due to be auctioned in September. A spokesperson for the auctioneers would not identify the owner of the jewellery, but said: "I have spoken to him and he is totally devastated, very upset.
"The money is not important to him, not the issue.
"He has asked me if there is any chance of the items being recovered and we are hopeful that they will be."
The exhibition is set to run until 14 September.