Nearly 31 million viewers watched live U.S. television coverage of Donald Trump's presidential inauguration, far fewer than tuned in to Barack Obama's first swearing-in, but otherwise the biggest such audience since Ronald Reagan entered office, ratings firm Nielsen reported on Saturday.

The tally for Trump - 30.6 million viewers on 12 broadcast and cable networks that aired live coverage from about 10 a.m. to 6 p.m. EST - surpassed the TV audiences measured for his two most recent Republican predecessors - both inaugurations of George W. Bush and the one of his father, George H.W. Bush.

The 2009 inauguration of Obama, who became the nation's first African-American president that year, was watched by nearly 38 million viewers, the second-highest number since Nielsen began compiling such figures with Richard Nixon's 1969 oath of office.

Only Reagan drew a bigger U.S. TV inauguration audience, with nearly 42 million viewers tuning in to see the California Republican sworn in for his first term in 1981. Just 25 million watched Reagan take his second oath of office four years later.

Trump's total was greater than both swearing-ins of Democrat Bill Clinton - 29.7 million and 21.6 million - and the second inauguration of Obama, who drew an average audience of over 20.5 million in 2013, Nielsen said.

George W. Bush holds the distinction for having the least-watched inauguration in Nielsen's historical data - 15.5 million viewers in 2005 after he defeated John Kerry for his second term in the White House. That was down sharply from the 29 million who tuned in for Bush's first oath in 2001, following his contested election victory over Al Gore.

The elder Bush by comparison averaged 23.3 million viewers.

Nixon, a Republican, was the only two-term president since Nielsen began compiling such records to draw a bigger audience in his second inauguration than his first, rising from 27 million in 1969 to nearly 33 million in 1973.

The only Democrat besides Obama to ever score more inauguration viewers than Trump was Jimmy Carter, with 34.1 million viewers watching his 1977 swearing-in, according to Nielsen.