Part of Oxford Street was closed after a suspected electrical explosion.
The Met Police said one man had been left with minor injuries after a small "power network explosion" at about 19:00 BST.
A police cordon was put in place blocking traffic and part of the pavement, but has since been lifted.
Eyewitnesses described "screaming, crying and shouting" after a loud explosion, followed by "heat and light" coming from a box of electrical wires.
Bronte Aurell tweeted: "I saw the explosion on #oxfordstreet I was right there - if that's an electrical explosion I don't want to ever meet one again. Was massive!"
Adam Jogee tweeted: "Terrifying few moments in John Lewis on Oxford Street. Explosion and lots of screaming, crying and shouting. All told to hide or get out."
Twitter account @Londonstuff tweeted a video showing a large amount of smoke saying: "Something's happened on Oxford Street. People running away quickly and panicking."
A spokeswoman from the Met confirmed roads had now reopened and emergency teams had stood down.
She said the electricity company was at the scene.
