Jeremy Corbyn said he would tackle Donald Trump on environmental issues if he was the Prime Minister attending the G20 summit.
If he were in Theresa May's place, he said he would also challenge the "economic orthodoxy" of austerity, which he said was creating inequality around the world.
The Labour leader was speaking in North East England, not Hamburg, ahead of addressing a large crowd at the Durham Miners' Gala, which he said was "Europe's biggest demonstration of working class culture".
Organisers said 200,000 people were expected for the traditional Big Meeting of brass bands and pit banners, which culminates in political speeches on the Old Racecourse.
Asked what he would be doing if he was Prime Minister at G20, he said:
"I would be very strongly challenging Donald Trump on his wish to walk away from the Paris Climate Change accords.
"They are crucial for the future of all of us and I hope he will understand that unless all the nations of the world get together to reduce emissions and try to preserve and protect our planet then the next generation are going to have more climate disasters, more extreme weather conditions and there's going to be more environmental refugees around the world.
"I think these are serious times and it requires serious attention to them."
Mr Corbyn, who watched some of the parade from the County Hotel balcony, praised the traditional Gala, which organisers said was the biggest since the 1960s.
"It's incredibly important for everyone to be here today.
"It is the most amazing demonstration of community strength.
"It's grown out of the struggles of the miners during the 19th, 20th and now in this century, it's a community event."
He refused to be drawn into a row about some local Labour MPs being "disinvited" from the event.
From the gala platform, he was expected to claim he is leading a "government-in-waiting" which could end austerity and scrap the cap on public sector pay rises.
But he will warn members that will only happen if the party can stay united.