DETROIT (AFP) - Republican front runner Donald Trump made one of the most off-colour remarks of his party's 11 presidential debates on Thursday (March 3), in a veiled reference to his own genitals within minutes of the showdown's opening.

His bombast has been a hallmark of the 2016 campaign from the moment he entered the race eight months ago, but he stooped to a new level at the GOP debate in Detroit, Michigan.

"He hit my hands. Nobody has ever hit my hands," Mr Trump said, referring to rival Florida Senator Marco Rubio's recent personal attack stating that the celebrity billionaire has small hands.

"Are they small hands?" he asked as he held up his two arms, to laughter and boos from the raucous debate crowd.

"He referred to my hands," he went on. "If they are small, something else must be small. I guarantee you there is no problem."

Mr Trump's remarks, likely unprecedented in a US presidential debate, appeared as the nadir of a campaign season already notable for its provocative and assaulting tone.

Mr Rubio excused his own attacks by essentially saying it was Mr Trump who first opened the flood gates.

"Donald Trump has basically mocked everybody with personal attacks," Mr Rubio said.

"If there's anyone who's ever deserved to be attacked that way, it was Donald Trump."

The two other candidates, Texas Senator Ted Cruz and Ohio Governor John Kasich, stood quietly as Mr Trump and Mr Rubio carried out their fiery exchange.