Nigel Farage has promised to "change politics forever" after his Reform UK party won more than four million votes, propelling him into Parliament for the first time.
Reform UK has won five seats in the House of Commons, with a 14.3% share of the vote.
Mr Farage overturned a Conservative majority of more than 25,000 to comfortably win in Clacton, Essex - a race which marked his eighth attempt to enter the Commons.
The election was the first step in a “mass movement across the country” that would "change politics forever”, he told supporters.
Mr Farage's first press conference as an MP was interrupted by six protesters who repeatedly accused the Reform leader of being racist and planning to sell off the NHS.
As Reform supporters removed the protesters one by one, Mr Farage suggested they could be paid actors aiming to disrupt his plans.
Mr Farage said: "The political establishment are in fear in private as to what happened last night with those results in the election."
When he returned to frontline politics and took over as head of Reform in June, Mr Farage said he had two aims - "first to get millions of votes" and the "second was to establish a bridgehead in parliament".
Mr Farage promised to now use his platform in Parliament and "work with anyone" to achieve his long-time aim of scrapping the First Past the Post Electoral system - highlighting the party's 14% vote share that yielded just five MPs.
"Above all what we're going to do from today is we're going to professionalise the party, we're going to democratise the party and those few bad apples that have crept in will be gone, will be long gone, and we will never have any of their type back in our organisation," he said.
Mr Farage denied he would join any pact with the Tories, saying he would prefer to "let the Conservative Party tear themselves apart".
"We're going to focus on going after the Labour vote" instead, he said.
As he was speaking, Mr Farage received a congratulatory message from former president Donald Trump.
In a social media post, Mr Trump wrote: "Congratulations to Nigel Farage on his big WIN of a Parliament Seat Amid Reform UK Election Success.
"Nigel is a man who truly loves his Country."
Reform also gained Great Yarmouth, Boston and Skegness, and South Basildon and East Thurrock from the Tories, while former Conservative MP Lee Anderson - who defected to Reform in March - retained Ashfield in Nottinghamshire.
The party drew large support in areas where the Conservatives won in 2019 under Boris Johnson, coming second in many constituencies.
An earlier exit poll for broadcasters had forecast the party would win 13 MPs, more than many polls during the campaign had predicted.
However, the figure was highly uncertain, as the model suggested there were many places where the party only has a relatively low chance of winning.
Polling expert Sir John Curtice said Reform had benefited from a significant fall in the Conservative vote in seats the party had previously held, as well as advancing most in areas where people voted Leave in the 2016 referendum.
In all four seats won by Reform, more than 70% of people voted for Brexit.
Reform UKs five MPs are:
- Nigel Farage, who won 21,225 votes - earning a 8,405 majority
- Richard Tice, the Reform UK chairman, who overturned a 27,402 Tory majority to win Boston and Skegness, beating the incumbent MP by 2,010 votes
- Lee Anderson won his Ashfield seat, defeating Labour by 5,508 votes and relegating his former party, the Conservatives, to fourth place
- Rupert Lowe, former Southampton FC chairman, who beat the Labour candidate by 1,426 votes in Great Yarmouth
- James McMurdock won in South Basildon and East Thurrock, after several recounts, with a majority of just 98 votes over Labour.
Reform UK candidates also came second in 98 constituencies.
In an early sign of Reform's success in winning over former Tory voters, the first two results of the night in north-east England - in Blyth and Ashington and in Houghton and Sunderland South - saw the party beat the Conservatives by more than 4,000 votes.
The pattern was repeated in a number of other seats, as the Tory vote share plummeted.
However, Reform had less success winning seats off Labour.
In Barnsley North, where the exit poll had forecast a 99% likelihood of Reform taking the seat, Labour held the seat with an increased majority of 7,811.
Reform's candidate, Robert Lomas, who was disowned by the party last week for offensive comments on social media, came in second place.
In Hartlepool, another seat forecast to go to Reform, Labour also held on comfortably with a majority of 7,698.
Mr Farage's surprise announcement that he was standing in the election, after previously saying he would not, saw Reform surges in opinion polls.
At the same time, he took over from Mr Tice as Reform's leader and he has played a prominent role in the party's campaign.
The former UKIP and Brexit Party leader has stood unsuccessfully to be an MP seven times, most recently in South Thanet, Kent, in the 2015 general election, when he finished second behind the Tory candidate.
Clacton was the first constituency to elect a UKIP MP in 2014, after former Tory MP Douglas Carswell defected to the party and triggered a by-election, which he won.
In 2019 Reform's previous incarnation, the Brexit Party, stood aside in more than 300 seats previously won by the Tories, amid concerns it could split the pro-Brexit vote.
However, this time the party contested 630 seats across England, Scotland and Wales.
Fielding an almost full slate of candidates in Great Britain posed challenges for the party.
Reform has had to disown six of them over offensive comments since nominations closed.
The party has blamed the surprise announcement of a July election, as well as claiming a company it hired to conduct background checks on would-be candidates failed to carry out vetting before the election was called.
Two Reform candidates also defected to the Conservatives over what they said was a failure of the party’s leadership to tackle the issue.
However, it was too late to remove any of these candidates so they still appeared for the party on ballot papers.