Operation Black Vote (OBV) has accused Zac Goldsmith of encouraging the "politics of division", as anger over his leaflets targeting ethnic minorities worsened.
The Tory mayoral candidate was criticised for sending leaflets to British Tamils warning that their family jewellery was at stake due to his opponent Sadiq Khan's policies. A separate leaflet sent out to British Indians said the Labour candidate was insufficiently supportive of Indian prime minister Narendra Modi.
Many have also complained about the Goldsmith campaign's insistence on linking the name Khan with the words 'radical' and 'dangerous', in what they suspect is an attempt to whip up Islamophobic sentiment.
"Goldsmith's team seem to have made the political calculation that by targeting certain ethnic and religious communities, they can sow the seeds of division and turn one community against another for political gain," OBV director Simon Woolley said.
"These leaflets and letters on their own can easily be explained away, but together they are sadly the politics of division and should stop immediately.
"This is not Donald Trump land where communities are pitted one against the other for political gain. This is London where we are proud of our diversity and that in spite of our many challenges, like no other teeming metropolis, we do get on with each other."
Woolley confirmed that OBV would be writing to all mayoral candidates asking them to sign a code of conduct which would rule out "personal attacks and divisive politics".
Tulip Siddiq, Labour MP for Hampstead and Kilburn, commented: "It is unprecedented for Operation Black Vote to criticise a political campaign in this manner.
"This is beyond party politics - it is a matter of basic decency. Zac Goldsmith should stop this negative and divisive campaigning immediately."
In fact, the Operation Black Vote move is not unprecedented. The organisation also warned of the rhetoric coming from Boris Johnson's camp during the 2008 mayoral election.
But it is bound to increase the pressure on the Goldsmith camp to water down some of the more fiery language being aimed at Khan.
Writing in the Daily Telegraph newspaper Tory councillor Binita Mehta said the approach would "turn off" prospective voters.
"Not all British Indians are fans of Modi – I have my reservations – nor do we particularly care about the current prime minister of the country our grandparents are from," she said.
"In pursuing the suburban Indian vote … it must be recognised that a blanket approach can seem stereotypical and patronising, and will certainly turn people off.
"I hate to have to say this but obviously we 'BMEs' are much more sophisticated than these targeted letters suggest."
Former Tory candidate Shazia Awan also lashed out at the leaflets.
Analysts believe the Goldsmith camp is trying to tailor a message to people wary of electing a Muslim mayor – particularly among other ethnic minorities and voters in outer London - without directly mentioning Khan's religion.
Asked about the leaflets by the Guardian newspaper, Goldsmith said:
"It’s not a race element at all. I talk to different communities about their concerns. When I have a public meeting with British Indians, they worry about what's happened to the Labour party – historically, these are people more likely to vote Labour than Conservative – and they're worried about things like wealth tax and the fact that jewellery burglary is going up and they are being targeted because they tend to have more jewellery in their homes than most other people."
Told that the words 'radical' and 'divisive' were "loaded" when referring to a Muslim, Goldsmith responded:
"Who's criticised me? Honestly, who's criticised me? He has. No one else has. Love it or hate it, the Labour party is more radical now than any time in my lifetime. He is radical and divisive because of his approach to politics. He is a fundamentally partisan figure in politics. These are terms that I use and will continue to use to describe Sadiq Khan. And it is obvious to anybody who is not unfair or wanting to misinterpret, the terms I was using were in a political context."
He also addressed comments concerning new taxes on family jewellery, telling Politics.co.uk:
"Can I reassure you whether you're a Tamil, whether you're a Hindu, whether you're a Sikh, whatever your ethnic origin or faith, I'm going to be a mayor for all Londoners and your gold is safe with me as the mayor."
The London mayoral election takes place on May 5th.