BNP leader Nick Griffin has been pelted with eggs and forced to abandon a press conference outside Parliament.
Dozens of protesters disrupted the event, which follows the British National Party winning its first two seats in the European Parliament.
Chanting anti-Nazi slogans and holding placards they surrounded Mr Griffin as he was bundled into a car.
Mr Griffin was elected for the North West region - a result condemned by parties across the political spectrum.
Mr Griffin and Andrew Brons, who was elected in the Yorkshire and Humber region, staged a press conference on College Green, opposite the Houses of Parliament.
The BNP leader began the event by holding up copies of national newspapers and talking about what he said were media lies about him and his party.
He denied he had past links with Oswald Mosley, as the former fascist leader "was very hostile to the National Front from which I am from".
He had been speaking for about two minutes when about 50 to 100 protesters marched towards him chanting slogans and throwing eggs.
Nick Griffin was forced to retreat from college green
There was a brief scuffle as Mr Griffin was jostled by protesters - and a scrum of cameramen - before the BNP leader was bundled into the back seat of a waiting car by his security men.
A tourist who was caught up in the melee was treated in an ambulance, after suffering an injured leg.
A spokesman for the Metropolitan Police said: "We are aware of the incident and will pursue charges if a complaint is made."
Members of Unite Against Fascism, a new group supported by trade unions and MPs from all parties, including Tory leader David Cameron, and veteran left wing campaigner Tony Benn, said they wanted to "defend democracy" against what they regard as the "fascist" and "racist" policies of the BNP.
One told the crowd his message for Mr Griffin was: "Wherever you go in this country we will make sure you are welcomed by demonstrations."
The demonstrators kicked and hit the BNP leader's car with their placards before cheering as he drove off.
Protest organiser Weyman Bennett, national secretary of Unite Against Fascism, said he believed it was important to stand up to the BNP.
"The majority of people did not vote for the BNP, they did not vote at all. The BNP was able to dupe them into saying that they had an answer to people's problems.
Nick Griffin: 'We were attacked by a mob'
"They presented themselves as a mainstream party. The reality was because the turnout was so low, they actually got elected."
Speaking to BBC News afterwards, Mr Griffin alleged that the three main political parties were trying to prevent the BNP getting its message across by colluding with protesters who he said were mainly left-wing students.
"It's a very, very sad day for British democracy," he said.
"People should be entitled to hear what we have to say and to hear journalists question us robustly."
He described protesters as an "organised mob that's backed by all three main parties to stop us getting our message across to the public" and added: "It does not represent ordinary people."
He said he suffered only a "glancing splattering" with egg but a television cameraman was hit full in the face.
He says the BNP is not racist and says it won votes because it "spoke openly about the problem of immigration".
And he argued that the "political elite" were responsible for making "the indigenous British majority... second class citizens in every possible sphere".
Mr Griffin said the BNP plans to hold a press conference in Manchester tomorrow and he hopes that the police will take action against any violent protests.
On Monday Tory leader David Cameron and Lib Dem leader Nick Clegg said they were "sickened" by the BNP's win and Labour deputy Harriet Harman said it was "a terrible thing".
The number of people voting BNP across the UK as a whole went up slightly, from 808,201 to 943,598 in the European elections, but went down slightly in the two regions where it gained MEPs, with the party benefitting from a collapse in the Labour vote.
They won 6.2% of votes, compared with 8.6% for the Green Party, 13.7% for the Lib Dems, 15.7% for Labour, 16.5% for the UK Independence Party and 27.7% for the Conservatives.
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