LOS ANGELES (Reuters) - Some 28 million viewers worldwide watched Oprah Winfrey's two-part interview last week in which cyclist Lance Armstrong finally admitted he had been taking performance-enhancing substances for years, Winfrey's OWN cable TV channel said on Tuesday.

OWN said that 12.2 million Americans and 15 million people overseas watched the encounter over two nights and various airings on OWN, with another 800,000 watching online on Oprah.com.

The two-part exclusive interview - totaling 2.5 hours - was seen in more than 190 nations in 30 languages, said the cable channel, a joint venture with Discovery Communications.

In the United States however, audiences shrunk dramatically after the first broadcast, in which Armstrong admitted in the first five minutes that he had been doping for all seven of his Tour de France victories.

Some 3.2 million Americans watched Thursday night's first broadcast, rising to 4.3 million including repeats. On Friday, just 1.8 million came back for part two, rising to 2.4 million including a later repeat.

Thursday's first interview failed to bring in record numbers for OWN despite days of advance publicity and interest from the cycling world. The most-watched telecast in OWN history was Winfrey's interview with the family of the late Whitney Houston in March 2012 that drew 3.5 million viewers.

Nevertheless, OWN said that Friday's broadcast marked the cable channel's highest-rated Friday telecast in its short lifetime to date.

Winfrey quit her daytime U.S. TV show in 2011 after 25 years to launch the Oprah Winfrey Network (OWN), a female-oriented lifestyle channel aimed at young women that has struggled to find its footing.

The Armstrong interview generated huge publicity for Winfrey and high demand for advertising that executives hope will translate into more regular viewers for OWN.