Nearly 23 million people watched Wednesday night's US presidential Republican primary debate, a record for the event's host CNN, the cable network announced.

"The Republican prime-time debate had 22.9 million viewers, making it the most watched program in CNN history," it said Thursday after billionaire Donald Trump, former Florida governor Jeb Bush and nine others clashed at the Ronald Reagan Presidential Library.

The figure nearly matched the 24 million viewers who tuned into Fox News on August 6 for the debut main debate of the 2016 Republican election cycle, by far the highest rating of any presidential primary debate.

The "undercard" debate featuring four lower-polling candidates drew an impressive 6.3 million viewers, the network said.

The record numbers made it clear that, despite the 14 months between now and the November 2016 election, Americans are eager to study the 15 main Republican candidates seeking to succeed President Barack Obama in the White House.

The next Republican showdown is set for October 28, while Democrats, featuring frontrunner Hillary Clinton and her chief rival, the liberal Senator Bernie Sanders, gather for their first debate on October 13.