By Peter Griffiths
LONDON (Reuters) - The leader of Britain's rising right-wing anti-EU party on Friday promised a political "earthquake" with victory in next year's European elections, a challenge that could threaten David Cameron's hopes of a second term as prime minister.
Nigel Farage, head of the anti-mass immigration UK Independence Party (UKIP), told the party's annual conference he planned to overturn decades of dominance by Britain's main three parties, the Conservatives, Labour and the Liberal Democrats.
But coverage of his speech was overshadowed by a senior party member's outburst and an altercation with a journalist.
Godfrey Bloom, a UKIP member of the European Parliament, described women at a political meeting as "sluts" and later hit a journalist over the head with a copy of a party brochure.
The incidents, which threatened to distract from Farage's political message, were unlikely to be welcomed by a party once derided by Cameron as full of "fruitcakes, loonies and closet racists" and now trying to portray itself as more mainstream.
UKIP has increased its support to around 10 percent, according to pollsters YouGov, after taking just three percent of the vote in the last national election in 2010 and failing to secure a single parliamentary seat.
Farage said if UKIP wins a majority of the UK seats in the 2014 election for European Parliament it would effectively be a condemnation of "open-door immigration" and Britain's membership in the 28-nation bloc, which the main parties generally support.
UKIP had a 16.5 percent share of the vote at the last European elections in 2009, securing 13 of Britain's 72 seats.
"We can come first and cause an earthquake," Farage said at the conference on the party's 20th anniversary. "We're changing the face of British politics."
Bloom said on Twitter he had been joking and had used the word "slut" in the sense of "untidy" to describe women who don't clean their kitchens thoroughly, rather than to mean they were promiscuous.
UKIP said it would exclude Bloom from the party, pending an investigation. In the meantime, Farage said his behaviour was "beyond the pale", but also offered some defense.
"He's not a racist, he's not an extremist ... he's not anti-women," he told the BBC. "But time and time again he says things that overshadow the whole agenda."
The party's fast growth has opened it up to closer scrutiny that has led to accusations of racism, right-wing extremism and homophobia.
Its founder Alan Sked, who left the party in 1997, said last year that UKIP had become "extremely right-wing", "morally dodgy" on immigration and lacked intellectual weight.
Farage stayed on theme on Friday, saying immigrants to Britain had caused a crime wave, milked the welfare system and strained demand for housing, healthcare and schools.
"I shall get severely criticized for this, but I have to say that there is an even darker side to the opening of the door," he said. "London is already experiencing a Romanian crime wave."
A former commodities trader who cultivates an image as a man of the people, posing for pictures with a cigarette and a pint of beer, Farage denies being racist or extremist.
British Influence, a campaign group that wants to keep Britain in the EU, accused him of scaremongering.
"Nigel is a demagogue, whipping up fear and hatred with a smile on his face and a pint in his hand," said Peter Wilding, the group's director.
UKIP secured nearly one in four of the votes cast at elections for local government jobs in May, buoyed by growing distrust of an EU many see as a threat to their sovereignty.
Surveys suggest it will do even better in the European elections next year. Under Britain's "winner takes all" system for national elections, it may still struggle to secure its first seat in the British parliament in the next vote in 2015.
(Editing by Sonya Hepinstall)