Hollywood star Mark Ruffalo has tagged US President Donald Trump the world's "public enemy number one", and urged people to "get radical" in their protests against the politician.

The "Apartment 12" actor, 52, has never made a secret of his dislike for Trump, and frequently slams him in posts on his social media pages, aceshowbiz.com.

However, during an interview with Sky News to promote his upcoming film "Dark Waters", Ruffalo took his campaign against the president to another level as he said there are no problems in the world bigger than Trump.

"I think the world should consider my president as public enemy number one at this point. What we do probably in the next ten years will be crucial to the future of the planet, and this is only gonna become more and more evident to us. We're not, we're not going backwards from here. And it's not just this story, this is a system," he said.

Asked if he believes there are any heroes in the world, Ruffalo said climate change activist Greta Thunberg, as well as praising radical protest group Extinction Rebellion.

In fact, it's the organisation's method of using disruptive demonstrations to promote their cause that the "Avengers: Endgame" star believes is the way forward.

"It's happening all over. And we're gonna have to make people uncomfortable. We're gonna have to get radical. We're gonna have to do peaceful, um, peaceful demonstration. Peaceful protest, peaceful actions that stop the system."