US President Barack Obama turns to look at the crowds gathered on the National Mall as he leaves the podium after his swearing-in ceremonies on the West front of the US Capitol in Washington, January 21, 2013. [Photo/Agencies]

Some 20.6 million Americans watched President Barack Obama's inauguration ceremony and related events on television, according to ratings data on Wednesday. That's down sharply from his first inauguration in 2009.

TV ratings company Nielsen said 18 US television networks and cable channels carried live coverage over about six hours of Monday's swearing-in ceremony, speech and parade in Washington.

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Monday's TV audience was a drop of 17.2 million from 2009, when 37.8 million Americans - the highest number since Ronald Reagan's 1981 inauguration - watched Obama formally take office as the first black president in US history.

The Nielsen figures did not measure viewers who watched Monday's daylong ceremonies online via live streaming on many TV channels, nor overseas audiences.

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Second-term inaugurations of US presidents have traditionally drawn smaller numbers of viewers than those for first terms.

Reagan's 1981 inauguration drew the biggest television audience of the past 44 years, attracting some 41.8 million US viewers, according to Nielsen.