LONDON (AFP) - Hundreds of demonstrators blocked a shopping mall in London on Wednesday to support "I can't breathe" protests that have swept the United States demanding justice for black men killed by police.

Police said that 40 people were cordoned outside the shopping centre after security guards were assaulted, and that they were in the process of making arrests.

Holding placards reading "Black lives matter" and "Hands up", the same slogans used in US protests, demonstrators lay down on the floor of the Westfield shopping centre in a mass "die-in".

Others walked with a banner reading "Solidarity with Ferguson" in reference to the Missouri suburb which saw riots after a decision not to prosecute a white officer who shot dead unarmed 18-year-old Michael Brown.

The crowd also chanted "We can't breathe", in reference to the last words gasped repeatedly by 43-year-old Eric Garner, a father of six killed in a police chokehold in New York in July.