Clint Eastwood's unconventional speech at the Republican National Convention in Tampa, Florida, has drawn a mixed reaction from the Hollywood community.

While endorsing US presidential hopeful Mitt Romney on Thursday, the 82-year-old at one point conducted an imaginary conversation with Barack Obama.

"What WAS that thing Clint Eastwood just did?" tweeted actress Mia Farrow. "How could they let it happen?"

Film critic Roger Ebert said the actor had come across as "sad and pathetic".

Adam Levine of pop band Maroon 5 suggested that the veteran Dirty Harry star had gone "nutterbutters".

Yet not everyone was as disparaging. "Clint Eastwood may be old but he's still awesome," opined former Saturday Night Live comedian Chris Kattan.

In a statement released after his appearance, the Romney campaign insisted the audience had "enjoyed" his "ad libbing".

"Judging an American icon like Clint Eastwood through a typical political lens doesn't work," his spokesperson said, claiming his contribution had been "a break from all the political speeches".

In general, though, the Hollywood community was less forgiving, with US actor Zachary Quinto purporting to be "legitimately scared" after his contribution.

"Is Clint Eastwood meant to instil confidence in Republicans?" mused Quinto, best known for his role as Mr Spock in 2009's Star Trek film.

"Apparently Clint Eastwood had an argument with an empty chair regarding its political standpoint," remarked Simon Pegg, another member of the current Trek crew.

"Maybe Clint is a sleeper agent for the Democrats sent in under deep cover to make the Republicans look stupid. No wait, that's Romney."

"Clint Eastwood became huge star as a man of few words," tweeted NBC veteran Tom Brokaw. "As a surprise guest on the Tampa stage he had too many words."

According to the Associated Press, the Oscar-winning film-maker and one-time mayor of Carmel, California, is "a fiscal conservative who takes left-leaning stands on social issues".

Eastwood will be back on the big screen later this year in Trouble with the Curve, playing an ageing, ailing baseball scout who embarks on a final recruiting trip.