He was there to make the Republicans' day. But the Hollywood star turned out to be less Dirty Harry than somebody's "drunk uncle Harry", as one Twitter contributor called him.

Clint Eastwood did end up stealing the show at Mitt Romney's formal appointment as his party's choice for the US presidential election but perhaps not in the way he or the candidate would have wanted.

The 82-year-old's rambling gravel-voiced conversation with an empty chair – supposedly supporting an invisible Barack Obama – proved a bizarre and confusing warm-up act for Romney.

"Mr President, how do you handle promises that you have made when you were running for election, and how do you handle them? I mean, what do you say to people?" he asked. He berated Obama for not learning from the Russian experience in invading Afghanistan. It was George W Bush who ordered US troops into the country.

The actor and director also talked about Oprah Winfrey, Obama's unfulfilled promise to close the US prison at Guantánamo, the war in Iraq and lawyers. He suggested it was a bad idea for lawyers to become presidents – Romney, like Obama, is a graduate of Harvard law school. Abraham Lincoln was also one.

At one point, he spoke about dismissing Obama and making a change.

"When somebody doesn't do the job, you've got to let them go," Eastwood said, drawing a finger across his throat. A speech that was supposed to last five minutes lasted nearly 12.

The Obama campaign hit back afterwards by tweeting a photo of the back of the president's chair, with Obama's head showing over the top of it, along with the line: "This seat's taken." Other tweets were ruder. "20 years ago I wanted Clint Eastwood to make my day. Now I just want him to take his pills and be grateful he doesn't need Medicare," said Nancy Lee Grahn, from the General Hospital television show.

"I can't believe I just watched Clint Eastwood turn into somebody's DRUNK UNCLE HARRY on the stage of the GOP RNC. He humiliated himself," said Star Jones, a lawyer and TV personality.

Among others who seemed to wonder if the actor was behaving like someone from another planet was George Takei – Sulu in the original Star Trek – who said he was, in response, "drafting a DNC speech to [an] imaginary Romney in an empty factory".

Romney's camp defended Eastwood. "He's an American icon," Romney spokeswoman Gail Gitcho told CNN's Piers Morgan. "You can't look at him at through the same political lens that you would other politicians. He's Clint Eastwood."