Respect Party candidate George Galloway has taken the Bradford West parliamentary seat from Labour, winning the by-election by 10,100 votes.
Mr Galloway, expelled by Labour in 2003, said it was the "most sensational victory" in by-election history. He received 18,341 votes - a 56% share.
He said his victory represented a "total rejection" of the major parties.
At the 2010 General Election, Labour's Marsha Singh, who resigned on health grounds, won with a majority of 5,763 .
The party had held the West Yorkshire seat since 1974, except for a brief period in the 1980s when the sitting MP defected to the SDP.
Labour candidate Imran Hussein came second with 8,201 votes as the party's share of the vote was 20% down on its 2010 figure.
'Bradford spring'
Conservative candidate Jackie Whiteley was third, with 2,746 votes. Jeanette Sunderland, of the Liberal Democrats, secured 1,505 votes.
Mr Galloway, who co-founded the anti-war Respect Party after being expelled by Labour because of comments he made as part of his opposition to the Iraq war, said the result represented the "Bradford