CLACTON-ON-SEA, England, Oct 10 (Reuters) - Britain's anti-EU UK Independence Party won its first elected seat in parliament on Friday by a wide margin.

In the English seaside town of Clacton-on-Sea, UKIP candidate Douglas Carswell won 21,113 votes or 60 percent of the vote, giving him a majority of 12,404.

Carswell, formerly the sitting parliamentarian from Prime Minister David Cameron's Conservative party, had triggered the by-election by defecting to UKIP.

Cameron's party came second with 8,709 votes or 25 percent of the vote. Labour came third with 3,957 votes. Turnout was 51 percent.

UKIP's success highlighted the threat it poses to Cameron seven months before a national election and its ability to split the mainstream Conservative party's vote casting a cloud over its re-election prospects in 2015.

(Reporting by Andrew Osborn; Editing by Guy Faulconbridge)