Justin Bieber singing outside Buckingham Palace. Picture: Screengrab/YouTube

Justin Bieber serenaded Hailey Baldwin while busking outside Buckingham Palace in London on Tuesday.

Onlookers were treated to an impromptu concert after the "What Do You Mean" hitmaker hopped out of the car he was travelling, with his guitar in-hand before taking a seat at the fountain and breaking out in song. Bieber dedicated his song "Coldwater" to Hailey.

A crowd started forming as news the celebrity couple's presence became known, with many taking to social media to share pictures and videos of the pair. One user wrote on Twitter: "So im just sitting on a fountain outside Buckingham palace when some big black cars stop right in front of me and some kid in a pink hoodie gets out with a guitar, turns out to be @justinbieber and he starts serenading his Mrs. (sic)"

He also claims Bieber asked for requests, but doesn't think anyone else heard.

Meanwhile, Hailey's uncle Alec Baldwin recently revealed that the pair tied the knot in New York.

Speaking at the Emmys on Monday Baldwin admitted he doesn't know "what the deal is" but claims the couple, who were spotted at the Marriage Bureau at the courthouse in New York City last Thursday, have tied the knot. 

He said: "They just went off and got married and I don't know what the deal is. We text Hailey every now and then. We met him one time."