(Adds online viewership)
LOS ANGELES, June 28 (Reuters) - The second debate among Democrats running for U.S. president attracted 18.1 million television viewers across three networks, a record audience for a Democratic primary face-off, NBC News said on Friday.
Thursday's event featuring front-runners Joe Biden and Bernie Sanders was shown on broadcaster NBC, cable channel MSNBC and Spanish-language network Telemundo, all owned by Comcast Corp.
The previous record for a Democratic debate was 15.8 million viewers, set in October 2015. Republican candidates set the all-time record for a primary debate audience in August 2015, when 24 million tuned in for the first performance by Donald Trump in a debate on Fox News Channel.
During Thursday's event, candidate Kamala Harris dominated her rivals and confronted Biden about his opposition to school busing in the 1970s.
An additional 9 million people watched live on Thursday via online streaming, NBC said.
The debate was the second of two with Democratic candidates aiming to challenge Trump. An estimated 15.3 million television viewers tuned in on Wednesday to watch Elizabeth Warren, Beto O'Rourke and eight other challengers, NBC said.
Reporting by Lisa Richwine; Editing by Sandra Maler, Lisa Shumaker and Richard Chang