Leading British politicians including Jeremy Corbyn and Sadiq Khan are among thousands of people who have signed a letter of support f the four US congresswomen of colour who were targeted by Donald Trump in a series of racist tweets this week.

The letter from the anti-racist charity Hope Not Hate expresses solidarity with Ilhan Omar, Rashida Tlaib, Ayanna Pressley and Alexandria Ocasio-Cortez, describing them as “the best of America”, and condemns Trump’s “blatant, unashamed racism”.

In less than two days the petition has been signed by dozens of MPs, two trade union general secretaries and 13,809 Hope Not Hate activists. The letter thanks the four congresswomen for “showing the world that America can still provide leadership to be proud of, even when the White House has abdicated that role.”

Khan, the London mayor, tweeted about the petition and said: “These progressive congresswomen represent hope for the future – their home is America, but their message crosses borders.”

Afzal Khan, a shadow immigration minister, said: “I have been inspired by the new generation of progressive US congresswomen. They are being threatened because they are a threat to Trump’s antiquated, racist views. In my view that is something to be proud of.

“Trump’s rhetoric reverberates across the world. Islamophobia is a growing problem in UK politics, and Trump has fed the fire with these remarks. Our message of solidarity is to these congresswomen, but also to any young person aspiring to a life in politics, to say we will not stand for racism.”

Matthew McGregor, a campaign director at Hope not Hate, said Trump’s tweets were a “disgusting attack on four congresswomen of colour” and he wanted the women to know that “from across the political spectrum in Britain, thousands and thousands of people are grateful for their leadership for the hope they give people, and that they have our solidarity.”

Trump doubled down on his comments during a rally in North Carolina on Wednesday, saying “if they don’t love it, tell ’em to leave it”. The crowd chanted “send her back”, in reference to Omar, who arrived in the US almost 30 years ago as a child refugee.