Some 80.9 million people watched the first presidential debate between Democrat Hillary Clinton and Republican Donald Trump on television in the US, setting a new viewer record for such events, CNN said on Tuesday.

The figure includes the approximately 45 million who watched the 90-minute debate on the four main TV networks, as well as millions more who watched cable channels and other smaller outlets, CNN said.

The figures, which are not final, beat the 80.6 million record set by the Jimmy Carter-Ronald Reagan TV presidential debate in 1980 but are below the 100 million viewers some analysts had predicted ahead of the face-off.

An audience of around 100 million would have ranked the debate in the league of the annual football Super Bowl, which attracts more than 100 million viewers and is the biggest TV event in the US.

CNN said viewership remained high throughout and, according to Nielsen data, there was not a big drop-off after the first hour of the 98-minute debate.

Monday's figures did not include millions of people who watched the debate in bars, restaurants and online through Twitter, Facebook and other social media. Monday's debate was the first of three between the presidential candidates before the Nov 8 election. The other two are on Oct 9 and 19.