Oh-saka: Fist-bumps and love-ins for Trump at G20

"Delinquents" one day, "terrific relationship" the next: Donald Trump has undergone a remarkable transformation from bashing the world's leaders at home to schmoozing happily like someone at a school reunion.

As if fundamental differences between the world's top powers over trade, foreign policy and the environment were not enough, Trump raised fears of an explosive summit by taking aim at all and sundry ahead of the trip.

But it was all smiles, praise and positive body language in person in Osaka as world leaders heeded Japanese Prime Minister Shinzo Abe's call for "beautiful harmony" -- a reference to Japan's new era name.

As leaders gathered for a group photo, Trump chatted warmly with Russian President Vladimir Putin, patting him gently on the back as they parted ways.

At a meeting with Abe and Indian Prime Minister Narendra Modi, Trump opted for a jovial fist bump instead of an awkward three-way handshake.

Controversial crown prince of Saudi Arabia Mohammed bin Salman greeted Trump with a warm smile, a robust handshake and a pat on the hand, with the two talking like old pals after the photo.

And journalists hoping for fireworks in Trump's many bilateral meetings were also disappointed with the US president seemingly on very best behavior at least for the public parts of his interactions.

Chancellor Angela Merkel, with whom Trump has clashed bitterly before, was "a fantastic person", "a fantastic woman" and "a great friend of mine".

He said he had a "very, very good relationship" with Putin, even risking a gag about alleged Russian interference in the US election.

Asked by a journalist whether he would warn Putin about influencing the presidential vote next year, he smiled and wagged his finger playfully at the Kremlin strongman, saying: "Don't meddle in the election, president, don't meddle."

And the bromance hit fever-pitch when he met Brazil's Jair Bolsonaro, who has been dubbed the "Trump of the tropics".

"He is a special man, doing very well, very much loved by the people of Brazil," Trump said, smiling broadly.

For his part, Bolsonaro told the US president: "I have been a great admirer of you for quite some time, even before your election."

Bolsonaro immortalized their encounter in a photo posted on his official Twitter account, showing him posing alongside Trump in the US leader's characteristic pose: a jaunty thumbs up.

- 'Very exciting day' -

It was all such a contrast from only two days earlier when he systematically shot down several of America's most cherished and long-standing alliances.

He mocked Tokyo and Berlin over defense -- calling Germany "delinquents" for not paying enough into NATO and calling into question the military ties with Japan that have stood since World War II.

"If Japan is attacked, we will fight World War III. We will go in and protect them with our lives and with our treasure," he told Fox Business Network before leaving for Osaka.

"But if we're attacked, Japan doesn't have to help us. They can watch it on a Sony television."

The barrage did not stop even onboard Air Force One as he tweeted about "unacceptable" Indian tariffs on US goods that he was determined to stop.

China also came under fire, with Trump issuing his now-standard threat to slap tariffs on all Chinese imports if no trade deal were struck in Osaka.

Contacts between Trump and Xi were brief on Friday with an "extended" meeting expected on Saturday.

But even here, Trump sounded upbeat saying he was looking forward to a "productive" meeting and a "very exciting day."