O2 reported on Twitter that the arena was not evacuated.
Hi there, to clarify the building has not been evacuated. We test our alarms weekly with a clear announcement that it is a test. Thank you
— The O2 (@TheO2) February 21, 2020
Earlier, eyewitness reported that an announcement was read out saying: "Due to an emergency it is necessary to evacuate the building."
The O2 arena is located in south-east London.
Earlier, London Euston station had been evacuated following reports of a stabbing incident.