The House of Commons reconvened on Tuesday, nearly two weeks after May 6 elections in which Cameron's Conservatives won the most seats, but were forced to strike a power-sharing deal with the third-placed Liberal Democrats.
Critics of the lower house of parliament's speaker, John Bercow, threatened to cloud the intricate ceremony by voicing opposition to his re-election, but their calls were ignored and he retook his seat.
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"This is a new era for our politics and something of a new start," said Cameron, speaking from the government dispatch box for the first time flanked by his deputy premier, Lib Dem leader Nick Clegg, and the new Foreign Secretary William Hague.
The new parliament - elected after a record number of MPs stood down amid an expenses scandal which rocked British politics last year - was a "chance for a new generation to show just how good this place can be," he added.
Labour in opposition
The Labour Party - on the opposition benches to the speaker's left for the first time in 13 years - vowed to keep pressure on Cameron's coalition.
"We all agree that we need strong and stable government. But we also should agree that we need strong opposition," said Harriet Harman, the acting party leader after former prime minister Gordon Brown stepped down last week.
MPs will begin swearing oaths of allegiance to Queen Elizabeth II over the next few days.
They must adapt to the first power-sharing government in more than six decades: Conservative and Lib Dem leaders have long baited each other in House of Commons debates, but will now have to sit together.
Every MP will also be under increased scrutiny in the wake of last year's expenses scandal, where the revelation that MPs claimed thousands of pounds for second homes, duck houses and swimming pools caused public outcry.
A record number of almost 150 MPs stood down before the election, some because it was time to retire but others because they were discredited in the row, which resulted in charges brought against three MPs and one peer.
Challenge to speaker
Tough new rules aim to ensure no repeat of the claims: MPs living within 32km of parliament cannot claim for accommodation; only one relative can be employed and from July all expenses claims will be made public.
The wave of MPs who quit has allowed a whole new generation of MPs to take over: out of 649 MPs elected, just over a third have never been MPs before.
Parliament has three female Muslim MPs for the first time among eight Muslim MPs. There are also a record number of ethnic minority MPs, but at 26 MPs, or four per cent, this is still short of reflecting British society.
The challenge to the speaker was an unexpected distraction: a speaker has not been voted out since 1835, and is usually re-appointed on the nod without a formal Commons vote at the start of each parliament.
But Bercow has drawn criticism because, although he sat as a Conservative MP, he was seen by some Tories as being close to Labour and the speaker is meant to be above party politics.
A small number of opponents shouted "no" when the question of whether he should resume his chair was asked. In theory only one MP needs to oppose the motion in order to force a full vote, but the "no" cries were ignored.
