Prime Minister David Cameron and his Deputy Nick Clegg were today sworn in as MPs in the new-look House of Commons.
Tory and Lib Dem ministers from the coalition Government were among the first batch of newly-elected MPs to take the oath or affirm in order to take their seats on the famous green benches.
Speaker John Bercow, who was yesterday re-elected to the office, was the first Member to be sworn in, followed by Father of the House Sir Peter Tapsell.
Mr Clegg, who is an atheist, chose to affirm rather than swear an oath on one of the holy books.
The process of swearing in MPs is expected to continue into next week.
More than a third of MPs - 226 of the 650-seat chamber - will be fresh faces in Parliament after the expenses scandal prompted the biggest exodus in living memory.
According to tradition, the Speaker is the first MP to take the oath, followed by the Father of the House, the Cabinet, the shadow cabinet and other privy counsellors and ministers.
Backbench MPs are taken in order of seniority, based on length of service in the Commons.
MPs can choose to swear on either the New Testament, the Old Testament (in English or Hebrew), the Koran, the Granth, the Welsh Bible or the Gaelic Bible.
Mr Bercow warned MPs that they would be on camera and their words could be picked up by microphones in the Commons chamber.
In 1997 former Labour minister Tony Banks was caught on camera with his fingers crossed when he took the oath of allegiance to the Queen.
Mr Bercow said: "I remind Members that the taking of the oath, as a proceeding of the House, is recorded by the television cameras and that anything said or done by Members may appear on television or be picked up by microphones."
The Speaker was re-elected without a vote yesterday despite objections from a small number of MPs.
There were cries of "hear, hear" from MPs as Mr Bercow detailed MPs' privileges, including the right to "freedom from arrest".
Mr Bercow restated his "absolute impartiality in this Chamber and beyond it" and his commitment to give backbench MPs "their rightful voice" in Commons proceedings.
Members who have not sworn in may not receive salaries, take their seats, speak in debate or vote.
They could be fined £500 and - more important - have their seat declared vacant "as if they were dead" if they attempt to do so.
Of the Cabinet who were present, all swore an oath of allegiance on one of the holy books as opposed to affirming. But Cabinet Office Minister Oliver Letwin chose to affirm.
There was a murmur among MPs as Labour leadership contender David Miliband was the first to step to the Despatch Box when the shadow Cabinet took their turn.
After affirming, he was followed by former Chancellor Alistair Darling and acting Labour leader Harriet Harman, who also chose to affirm.
The vast majority of shadow Cabinet members present chose to affirm, although former Cabinet Ministers Ben Bradshaw and Shaun Woodward were among those who swore an oath of allegiance on one of the holy books. Former Transport Minister Sadiq Khan swore his oath of allegiance on the Koran.
Labour Privy Counsellors Alun Michael and Ann Clwyd were both sworn in as MPs in Welsh.
Barnsley Central MP Eric Illsley, who was today suspended from the Labour Party, chose to affirm.
Mr Illsley is accused of dishonestly claiming more than £20,000 in expenses. He is the fifth politician to face criminal charges after last year's expenses scandal, and will appear at City of Westminster Magistrates' Court next month.
After being sworn in as an MP, Mr Illsley had a one-minute conversation with Mr Bercow in the Speaker's Chair, holding his handshake throughout.
Labour's Stephen Pound (Ealing N) crossed himself after swearing his oath and then kissed the Speaker's hand.
Lib Dem Andrew George (St Ives) took the oath in Cornish. The proud Cornishman led the parliamentary campaign to have the language officially recognised.
He claims to have been the first MP to speak Cornish in the Chamber when he made his maiden speech in 1997.