Smith performed hits "Men In Black" and "Summertime," in addition to his new single, "Switch" from the "I, Robot" soundtrack, Wednesday at Leicester Square, BBC reported Thursday.
"I wanted to give the people who came to support me a little treat out here," Smith said of his mini concert.
Smith, who has had a successful music and film career with movie hits "Men In Black" and "Independence Day," said music is how he expresses himself.
"You know as an actor I am more a tool for the director to see his or her vision, but as a musician I write what I want to write and say what I want to say," he said.
Smith stars as a detective in the film, which gave the actor the biggest U.S. box office opening in his career with a first weekend take of $53.3 million.
The action film opens Friday throughout Britain.