The showdown between Hillary Clinton and Donald Trump was the most-watched presidential debate ever, with 84 million viewers.
The Nielsen company said the viewership, over 13 different networks, toppled a record that had stood for 36 years. The previous record for presidential debate viewership was the 80.6 million people who saw the only debate in 1980 between incumbent Democrat Jimmy Carter and his Republican challenger Ronald Reagan.
At the time of the Carter-Reagan debate, the U.S. population was 226 million. Now, it is 324 million, according to the U.S. Census Bureau.
No debate since then had exceeded 70 million viewers.
Social media was humming, too, with Nielsen saying there were some 17.1 million Twitter interactions involving 2.7 million people on Monday. Tivo said that the moment during the debate that caused more people to pause their television and play back what was said came near the end, when Trump said that he will "absolutely support" Clinton if she is elected president.
Clinton has some bragging rights at home. When final results are in, the audience for her first presidential debate will more than double what her husband, former President Bill Clinton, received for his last presidential debate in 1996 (36.3 million viewers).
Only the Super Bowl annually commands a television audience of that size. The biggest audience in U.S. television history was the 114.4 million people who watched the 2015 Super Bowl between New England and Seattle.
The news was particularly good for NBC. Not only did it have more viewers than any other network showing the debate, but "Nightly News" anchor Lester Holt's reviews as moderator were more positive than Matt Lauer received for his interviews with the candidates at a national security forum earlier this month, or CNBC anchors when they did a GOP debate last fall.
Watching the debate was nerve-wracking for NBC Universal chief executive Steve Burke because of the pressure on Holt. Burke said at an appearance in London on Tuesday that Holt "ended up doing a very good job."
Holt was not available for an interview on Tuesday.
Some Republicans were unhappy with Holt, suggesting that he was unfair because he asked tougher questions of Trump, and challenged his facts on issues like Trump's support for the war in Iraq and a court case involving the "stop-and-frisk" method of policing.
That may account for an overnight change of thinking by the candidate. Interviewed by reporters immediately after the debate, Trump said that "I thought Lester did a really good job" and that he thought Holt brought up the topics he wanted.
Several hours later, on Fox News Channel's "Fox & Friends," Trump said he thought Holt earned a C or a C-plus for his debate performance, and that he asked unfair questions.
The second of three scheduled debates will be Oct. 9. The "town hall"-style forum will be moderated by CNN's Anderson Cooper and ABC News' Martha Raddatz.
Associated Press writer Jill Lawless contributed to this report.