Arnold Schwarzenegger hailed British troops as the "true action heroes" today.
The governor of California joined Prime Minister David Cameron on a visit to Wellington Barracks, near Buckingham Palace, where he met soldiers from 1st Battalion the Grenadier Guards.
He told the troops: "I get hailed a lot of times when I travel around the world as being the action hero.
"But I know the difference between a movie action hero who is make-believe and the true action heroes.
"You all are the true action heroes because you are risking your life.
"You are risking your life every day when you go out to the front so I say thank you, thank you, thank you for the great work that you are doing."
Speaking in front of about 300 troops, Mr Schwarzenegger said: "I'm always interested to hang out with heroes."
He paid tribute to the UK armed forces for being "great partners" with the US and doing a "great job".
He asked what made the two countries great, with freedom, economic prosperity and opportunities.
And he said: "It's because you, for hundreds of years, have fought for the freedom, to keep these countries free and to keep these countries strong and to have the freedom of speech. Many countries in the world don't have that."
The former Mr Universe said he had walked about the barracks and visited the gym.
He said: "First of all, I was amazed when I saw your guys pumping up in the gym with those deltoids and those biceps and the six-pack. Wow, you guys are really in great shape."
But he said he then went to the rehabilitation centre where he met a man who had a cage around his lower leg with pins through it who was undergoing months of intense recovery treatment.
He said: "That, to me, is really tough and brave. To make this mental adjustment and to go through this pain and torture and through the rehabilitation training, I admire that.
"This is the kind of strength I see in each and every one of you when I look in your eyes."
The battalion returned in April from a six-month tour in Afghanistan, during which five of their men died.
Mr Cameron said the men had a "very tough tour" in Afghanistan but faced it with "incredible bravery and dedication".
He continued: "We are not in Afghanistan to build the perfect democracy.
"We will never leave behind a sort of Switzerland in the Hindu Kush but we can make that country safe from terrorist training camps, which will make us safe back here at home.
"I think the British public understand that and I think we have the strength of character to see this through and to see it through successfully."
He said he wanted to "celebrate the incredibly close relationship between Britain and America".
He continued: "I think you saw that for yourself in Helmand Province where there are now 10,000 British forces and 20,000 US Marines and I could see how closely we were working together."
He said he was in regular contact with President Barack Obama, adding: "We are absolutely united in making sure we make Afghanistan safe from terror and that will make us safe from terror."
Introducing Mr Schwarzenegger, star of films including The Terminator and Commando, the Prime Minister said: "He's made a few movies which have involved a fair amount of what I think you would call kinetic action.
"I suspect you've watch a few of them."
Earlier, the two men stood side by side to watch the changing of the guard.
The governor, wearing a bright pink tie, was slightly shorter but also noticeably broader than Mr Cameron.
Surprised tourists clamoured at the railings to get a view and take photos.
After a private meeting with Lieutenant Colonel Roly Walker, Major Andrew James and Major Vince Gaunt, the pair joined soldiers in the canteen during lunch.
Some of the young soldiers gathered around to take pictures, with one saying: "That's going up on my Facebook page."
Before the visit to the barracks, the men met in Downing Street where Mr Cameron joked that the former action hero was going to help him "terminate" the UK's budget deficit.
As conservatives who have each tried to broaden their appeal by looking at environmental and social issues, the two men have struck up a firm friendship in recent years.
As Mr Cameron welcomed Mr Schwarzenegger to Number 10, the PM joked: "He's going to help me terminate the budget deficit."
Taking advantage of a visit to London to renew their acquaintance, the pair held brief talks in the White Room, where dealing with deficits was among the topics.
The outgoing governor unveiled a budget package last week aimed at closing a 19 billion US dollar (£12 billion) shortfall - representing more than 20% of the giant US state's spending.
It avoided tax rises by imposing cuts and delaying corporate tax breaks but was criticised for relying on over-optimistic predictions of future revenues.
Mr Cameron, who had been due to host a party for Baroness Thatcher's 85th birthday tonight, was also overheard telling Mr Schwarzenegger it was "quite a day" as he was meeting "the Iron Man and the Iron Lady".
Lady Thatcher later pulled out of the Downing Street reception after being taken ill with flu.Reuse content