Mr Trump opted to not participate in the debate amid a feud with Fox News
US presidential hopeful Donald Trump has been mocked by his Republican rivals after he boycotted the final party debate days before voting begins in the state-by-state contests to choose delegates.
Texas Senator Ted Cruz, who is locked in a tight contest with Mr Trump in Iowa, opened the debate with an impression of the real estate mogul's frequent attacks on his opponents.
He said: "I'm a maniac and everyone on this stage is stupid, fat and ugly. And Ben, you're a terrible surgeon.
"Now that we've gotten the Donald Trump portion out of the way..."
Former Florida Governor Jeb Bush, who has been frequently targeted by Mr Trump, added: "I kind of miss Donald Trump; he was a teddy bear to me."
The debate comes just four days before the Iowa caucuses kick off the 2016 nominating contest.
Residents will choose among the Republican and Democratic candidates battling to become their party's 2016 presidential nominee.
Mr Trump pulled out of the debate earlier this week, claiming he had been treated unfairly by host Fox News amid a months-long feud with anchor and debate moderator Megyn Kelly.
Ms Kelly began the debate by asking the candidates to address "the elephant not in the room".
The tycoon held a rally at the same time as the debate in Des Moines, which his campaign said was to raise money for military veterans.
Mr Trump claimed Fox News Channel apologised over the feud before the debate.
Fox News chairman Roger Ailes "acknowledged his concerns" about a statement the network made in the days leading up to the debate, but said Mr Trump had demanded a $5m contribution to his charities to appear at the debate. The network rejected that demand.
His absence saw the spotlight shift towards policy as Mr Cruz, as Florida Senator Marco Rubio, clashed on immigration.
Mr Cruz accused Mr Rubio of making a "politically advantageous" decision to support a 2013 Senate bill which included a pathway to citizenship, while the Florida senator claimed his rival was "willing to say or do anything to get votes".
Kentucky Senator Rand Paul - who did not qualify for the previous debate because of low poll numbers - was returned to the main stage.
The other candidates were retired neurosurgeon Ben Carson, Ohio Governor John Kasich and New Jersey Governor Chris Christie.
Some of Dr Carson's comments during the debate were highlighted by US media, including his claim that "Putin is a one-horse country: oil and energy".
In his closing statement, he also recited a chunk of the US Constitution, suggesting "folks, it's not too late. Enough said".
Four other candidates - former Hewlett Packard chief executive Carly Fiorina; former Arkansas Governor Mike Huckabee; former Pennsylvania Senator Rick Santorum and former Virginia Governor Jim Gilmore took part in an early undercard debate.