'Leave Greenland alone': Heckler disrupts US national anthem during London NBA game

This outburst happened as American singer Vanessa Williams was singing the "Star-Spangled Banner" at the O2 Area on Sunday before the Orlando Magic and Memphis Grizzlies game.

He yelled "Leave Greenland alone", before the arena erupted into cheers and whistles.

This came hours after Mr Trump announced a wave of tariffs on eight European countries, which would remain in place until he had acquired Greenland.

Read More: 'I no longer feel an obligation to think purely of Peace,' warns Trump after being refused Nobel Peace Prize

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The US president said the UK will be charged a 10 per cent tariff “on any and all goods” sent to the US from February 1, increased to 25 per cent from June 1, until a deal is reached.

Sir Keir Starmer described the threats from Mr Trump as a "moment for the whole country to pull together" as he blasted the US President for his aggressive stance, in an emergency speech from Downing Street.

Speaking to the nation on Monday, Sir Keir said: "The UK and the US are close allies and close partners.

"That relationship matters profoundly, not just to our security but to the prosperity and the stability that people here depend upon.

"Under President Trump, as under previous presidents, we are determined to keep that relationship strong, constructive and focused on results."

He added: "Mature alliances are not about pretending differences don't exist. They are about addressing them directly, respectfully and with a focus on results.

"On Greenland, the right way to approach an issue of this seriousness is through calm discussion between allies."

Mr Trump sent an extraordinary letter to the Norwegian Prime Minister saying he will focus on “what is good and proper for the United States of America,” after he was not awarded the Nobel peace prize.

He warned that he “no longer feels an obligation to think purely of peace” after being denied the prize.

He also issued a warning over Greenland saying the US needs to have “Complete and Total Control” in order for the world to remain secure, demanding Greenland is handed to America because Denmark can't protect it.

“Considering your Country decided not to give me the Nobel Peace Prize for having stopped 8 Wars PLUS, I no longer feel an obligation to think purely of Peace,” Mr Trump said.