At least seven people were injured when part of a cliff face broke away at the popular Navagio beach on the Greek island of Zakynthos, officials say.
British woman Wendy Thorpe said she was on a boat with her daughter when the cliff collapsed and there was an "almighty crash".
Navagio Beach, nicknamed Shipwreck Beach, is a popular tourist attraction in Zakynthos. At least seven people received minor injuries.
None of the injuries sustained in the incident on Thursday afternoon are believed to be life-threatening.
Footage posted on social media showed people running in panic as a large section of the cliff peeled off and crashed down close to several bathers.
O'Connell described the piece of rock that fell as being as big as a four-storey building, crashing down onto the beach.
A couple of small vessels overturned from the force of the fall and the subsequent high waves. While so far, no one has been found, they said they were continuing their search. Greek authorities said one foreign woman had been injured and taken to hospital.
Rescue services with sniffer dogs are searching the beach for possible victims.
Two small boats just off Navagio beach - nicknamed "Shipwreck Beach", after a freighter shipwreck in 1980 - also capsized as a result of the rockslide. The spot is popular with tourists and locals, who get to the cove either on small tourist or private boats.