Nick Griffin abandoned his news conference after protestors threw eggsUnite Against Facism who organised the demonstration, says one of the group was hit when a car, driven by a BNP member, "drove at" two protesters.
A Scotland Yard spokeswoman said officers had received an allegation of common assault and were investigating reports of a road collision.
Mr Griffin told SkyNews it was he and his colleagues who had been subjected to "serious physical harm" by the protesters.
"I make no apologies for having a small group of security," he said.
"I would rather it was done with the police, but the police, no doubt on orders from the Home Office, stood on the other side of the street.
"If, in their haste, one of the mob got in front of our car, I'm afraid that is their problem, not ours."
Nick Griffin was forced to abandon a news conference outside Parliament after the disruption by UAF.
The newly-elected MEP for the North West of England, arrived on College Green in front of Parliament with fellow BNP MEP Andrew Brons just after 2.30pm.
Mr Griffin started by attacking articles from today's newspapers that criticised him and his party.
He had only been speaking for a few minutes when the protesters appeared, chanting and waving banners reading: "Stop the fascist BNP."
Eggs were thrown at Mr Griffin and his bodyguards bundled him away through the crowd.