Nigel Farage hails the vote "the best night in UKIP's history" as he warns the Tories and Labour: "You have underestimated us."
UKIP leader Nigel Farage has warned David Cameron and Ed Miliband "we're after you" after the party claimed its first elected MP by winning 12,404 votes in the Clacton by-election.
Elsewhere, the party slashed Labour's majority in Heywood and Middleton to just 617 votes, which Mr Farage hailed an "absolutely astonishing" result.
Former Conservative MP Douglas Carswell won a landslide victory in Clacton, Essex, by securing 21,113 of the 35,386 votes cast.
UKIP also forced a recount in Heywood and Middleton, where Labour's Liz McInnes narrowly defeated UKIP's John Bickley.
Mr Farage said the results showed UKIP was now "a truly national party - indeed we are the only party that can challenge in solid Tory and Labour areas".
He went on: "I've no doubt we'll hear, 'it's just a protest vote'. It's nothing of the kind. People from across the spectrum are saying. 'we've had enough of career politics'. They want to have a proper voice in Westminster.
"From today with Douglas Carswell they have got one, and in a few weeks time I believe they will have another in the shape of Mark Reckless, with another by-election coming up in Rochester and Strood.
"What we saw last night was the biggest and best night in UKIP's history
"We will now take the people's army of UKIP to the Rochester and Strood, and we will give that absolutely everything we have got."
Mr Farage earlier told Sky's Faisal Islam the strong UKIP result showed the Eurosceptic party was "ripping lumps out of the old Labour vote in the north of England".
Asked if he had a message for both Mr Cameron, who turned 48 yesterday, and Mr Miliband, he replied, "we're after you", adding: "Happy Birthday to David Cameron, and Ed Miliband, you have underestimated us."
Mr Cameron said UKIP's success in Clacton and better-than-expected showing in Heywood and Middleton both spoke to a "wider truth" that a vote for UKIP risked a Labour government.
"If you vote UKIP you are in danger of getting a Labour government with Ed Miliband as Prime Minister, Ed Balls as Chancellor, and you'll get no action on immigration, no European referendum, and most importantly you won't get a continuation of the plan that's delivering success for our economy and security for our people," the Prime Minister said.
"That is the wider lesson of last night and we have several months to demonstrate that only a Conservative government can give people the stability and security that we all want to see."
Mr Miliband, who would not be drawn on the nail-biting recount at Heywood and Middleton, said: "There won't be a shred of complacency from us as we reach out to all of those voters who didn't vote Labour and those who didn't vote at all."
He said the 2015 election was a "fight against disillusionment and despair about politics" which he was determined to win.
Mr Carswell, who triggered the Clacton by-election when he defected from the Tories, said UKIP's success revealed a "profound change in British politics".
Ms McInnes, who gained 11,633 votes in Heywood and Middleton where voter turnout fell to just 36%, said the by-election result was a win for the NHS.
Analysts said Labour's vote was damaged by the low turnout. Voter turnout was higher in Clacton, where 51.2% of voters cast their ballot.