Unite Against Fascism (UAF) accused the BNP of exploiting a “toxic political climate” by planning a rally outside Lunar House, the Home Office’s main immigration centre.
The protesters said the far-right party was “trying to intimidate new migrants in our community”.
UAF joint secretary Sabby Dhalu said the rally was a “desperate attempt by the party to gain publicity”, as it struggles to recover from a rapid decline in membership since 2010.
Ms Dhalu claimed the rally was a strategic attempt to stir up anti-immigrant sentiment after a summer of headlines dominated by the European migrant crisis and delays at Calais ferry port.
She added: “It is something we expected. At the moment there is a bit of a toxic political climate, not helped by comments such as David Cameron describing migrants as a ‘swarm’.”
On YouTube, Croydon and Sutton BNP chairman Clifford Le May compared his party’s anti-immigration stance to the actions of Francis Drake, the Duke of Wellington, and citizens who took part in the Second World War.
Mr Le May said: “The Duke of Wellington was never called a racist when he stopped Napoleon’s plans to invade Britain.
“We are not thugs, we are not racist, we are ordinary patriotic British people.”
The rally at Lunar House will be met by a counter-demonstration organised by the UAF and trade unionists.
But Ms Dhalu said she expected the day to be peaceful, and dismissed the idea of allowing the BNP to march unopposed.