Eastwood's rambling, unscripted address at Thursday's convention to an absent President Barack Obama in an empty chair inspired an instant satirical Twitter account, @InvisibleObama, that quickly went viral, demonstrating the power of social media to upset tightly scripted image control.

Although Romney notched up the most tweets during his keynote address to the convention in Tampa, Florida - more than 14,289 tweets per minute - his Twitter Political Index (Twindex), which measures how tweeters feel about a candidate on a scale of 1 to 100, fell from 46 to 38 following his speech.

And by Friday, it was "Dirty Harry" star Eastwood's performance that was capturing the popular attention. The Twitter hashtag #eastwooding - mostly pictures of empty chairs - was also one of the top-trending topics on the microblogging site on Friday morning.

Paul Levinson, professor of media and communication studies at Fordham University and author of the book "New New Media," thought Eastwood's performance was "the biggest story by far from the convention, including Romney's speech."

"I don't think what happened with Eastwood will be decisive in the presidential election, but I think that forever and anon, when people think about this convention, they are going to think about this empty chair and this octogenarian actor rambling on," Levinson told Reuters.

The @InvisibleObama parody account garnered more than 25,000 followers by the end of Romney's speech, and by Friday morning it had more than 48,000 followers.

Eastwood's address was also an instant hot topic on political blogs and on television following Romney's address.

'A HORRIBLE BLUNDER'

CNN's Wolf Blitzer called it embarrassing and "a horrible blunder" by the Republican convention planners, while liberal-leaning MSNBC anchor Ed Schultz predicted that "tomorrow around the water cooler, it's all about Clint Eastwood. He's the big winner tonight."

Conservative-leaning Fox News Channel lingered on TV images of Romney's and vice-presidential candidate Paul Ryan's many children and grandchildren playing happily with some of the tens of thousands of red, white and blue balloons released at the end of the evening.

But anchor Megyn Kelly also opined that "a lot of people will be talking about Clint Eastwood."

Marty Kaplan, professor of politics and pop culture at USC's Annenberg School, said Republican planners were likely regretting they had invited Eastwood to speak.

"They're having to spend a huge portion of the time that ought to be a celebration of (Romney's) convention, and instead they're doing damage control. It's a distraction and I can't imagine they're happy about that," Kaplan told Reuters.

Perhaps fortunately for Romney, broadcast television audiences for Thursday were down sharply from the 2008 Republican convention, when little-known vice presidential candidate Sarah Palin captured the public imagination.

Early figures showed some 11 million Americans watched Romney (and Eastwood's) speech on ABC, NBC and CBS, in line with previous nights this week. NBC suffered the biggest audience drop - 76 percent - compared with the third night of the 2008 Republican gathering.

Figures for cable news outlets will be released later on Friday. But according to Nielsen data, total TV audiences for Wednesday night, when Ryan spoke, were 22 million - a 41 percent or 15 million drop from the equivalent night for the 2008 RNC when Palin made her entry onto the national stage.

The vast majority of viewers this year are aged 55 and over, according to Nielsen. Male-female breakdowns were not available but according to a CNN-Facebook social media partnership, more females were discussing the Republican convention than men in the last 24 hrs. A rudimentary graph can be viewed on the website cnn.com/election/2012/facebook-insights/.

In a world of political advertising, image control and political spin, the power of social media as exemplified by the Eastwood parodies was "a very healthy thing for democracy," Levinson said.

"You can't program social media. You can put up YouTube videos and set up Twitter accounts and Facebook pages but there is always something unpredictable that goes viral and that carries the day as to what the public takes away," he said.

Kaplan described Eastwood's appearance as "the juiciest thing" to come out of the convention. "When you use pop culture and Hollywood in those kind of figures, you're licking the razor, you're taking a risk, and politics, to some degree, is about controlling risk," he said.

(Reporting By Jill Serjeant and Piya Sinha-Roy; editing by Todd Eastham)

• A Permanent Economic Emergency