UKIP's Douglas Carswell taking Commons seat
UKIP's first elected MP said that "change is coming" as he arrived to take his seat at the House of Commons.
Douglas Carswell, who defected from the Conservatives, won last week's Clacton by-election by 12,404 votes.
Arriving at Westminster, Mr Carswell said he would focus on two things - bringing about a recall bill for MPs, and reforming the banking industry.
He told journalists he felt "reinvigorated" and paid tribute to his party leader Nigel Farage.
Labour's Liz McInnes, who won the by-election in Heywood and Middleton, was also sworn in as an MP and took her Commons seat for the first time.
MPs are returning to Westminster after the party conference recess.
Mr Carswell said people felt "let down" by what he called the "three-party system".
He said he would be supporting Conservative MP Zac Goldsmith's proposals for voters to be able to sack MPs, and said he would "hold ministers to account" over border control.
He also said the "Tory attack machine" had tried to undermine former Conservative MP Mark Reckless, who has also defected to UKIP, triggering a by-election in his Rochester and Strood constituency.