Officers were called to the shop shortly before 11:00 GMT as the raid was in progress, the force said.
Det Ch Insp Scott Mather, from the Flying Squad, said: “This robbery was carried out in a busy part of London, and we understand the alarm it has caused. Our officers are working to identify and arrest those responsible.
“Officers were on scene within minutes and investigators are working at pace to piece together the offenders’ movements.
“We are determined to find those involved and bring them to justice as swiftly as possible.”