Donald Trump loomed large over the final Republican debate before the Iowa caucuses, despite not being on stage.

His absence was mocked by his rivals, who tried to fill the space vacated by Trump's boycott by attacking each other on immigration and other issues.

Trump decided to withdraw after Fox News refused to drop debate host Megyn Kelly, whom he accused of bias.

The billionaire held a rally nearby, in honour of war veterans, that threatened to overshadow the debate itself.

On Monday, voters in Iowa are due to pick their presidential nominee for each party.

Days ahead of that critical test, Trump's absence on the stage in Des Moines was keenly felt by his seven rivals in the race to be Republican presidential nominee.

Texas Senator Ted Cruz addressed it with humour in the opening minutes by throwing mock insults at his rivals.

"I'm a maniac and everyone on this stage is stupid, fat and ugly, and Ben [Carson], you're a terrible surgeon," he said.

Former Florida Governor Jeb Bush also poked fun at the hotel tycoon, his chief tormenter in previous debates, by saying how much he missed him.

The debate as it happened

Image taken from BBC Online

Other highlights:

- Florida Senator Marco Rubio stood by a previous pledge to shut down mosques where radicalisation is taking place

- He also promised to tear up the nuclear deal with Iran on "day one" of being president

- Cruz was booed when he accused the Fox presenters of encouraging attacks on him

- New Jersey Governor Chris Christie said he knew nothing about the infamous, politically-motivated traffic jam ordered by his aides

- Kentucky Senator Rand Paul raised concerns about the US expanding military role in Syria

Elsewhere in Des Moines at the same time, Trump led a raucous rally in honour of the country's war veterans.

"When you're treated badly, you have to stick up for your rights," he said, referring to his row with Fox.

The broadcaster released a statement that said Trump offered to appear at the debate if Fox contributed $5m (£3.4m) to his charities, but they refused.

working hed: "TRUMPLESS AND NEWSLESS"

— Mark Halperin (@MarkHalperin) January 29, 2016

Data released by Google after the debate suggested that search interest in Trump still far surpassed the other candidates.

Many observers on social media thought the event was duller without the brash New Yorker.

But others remarked how the absence of his dominating personality helped other candidates to blossom.

Seriously, Bush is 500% better when he doesn't have Trump dunking his head in the toilet https://t.co/Wq53sls97e

— Jonathan Chait (@jonathanchait) January 29, 2016

Some of the night's most heated moments came during exchanges about immigration.

Cruz and Rubio were both forced to explain video clips of previous statements that appeared to be at odds with their hardline campaign pledges.

They then turned on each other, with Rubio saying immigration was "the lie that Ted's campaign is built upon".

An Iraq war veteran who came to the US from Mexico as a child appeared via YouTube to tell the candidates that "some of the comments in this campaign make us question our place in this country".

Bush applauded Dulce Candy and said "we should be a welcoming nation".

The Iowa caucuses on Monday are seen as the first real test of the election campaign, and the beginning of a series of state-by-state contests to choose delegates for both Republicans and Democrats.

Unlike a primary, which is a traditional election featuring secret ballots on polling day, the caucuses in Iowa are meetings of registered party voters and activists where they discuss the candidates and then vote.