Sean Penn has launched a veiled attack at US President Donald Trump in his debut novel, declaring ''we are a nation in need of an assassin''.

The 'Gangster Squad' star is publishing 'Bob Honey Who Just Do Stuff' - an expanded version of a 2016 audiobook he wrote under the pseudonym Pappy Pariah and narrated - and has added a new scene in which protagonist Honey writes a letter to the story's President named Mr. Landlord.

Penn's fictional letter reads: ''Many wonderful American people in pain and rage elected you. Many Russians did, too. Your position is an asterisk accepted as literally as your alternative facts. Though the office will remain real, you never were nor will be. A million women so dwarfed your penis-edency on the streets of Washington and around the world on the day of your piddly inauguration ... You are not simply a president of impeachment, you are a man in need of an intervention. We are not simply a people in need of an intervention, we are a nation in need of an assassin ... Tweet me bitch, I dare you.''

The 57-year-old actor is a committed Democrat supporter and an outspoken critic of President Trump, dubbing him ''an enemy of mankind'' in an opinion piece he wrote for Time magazine in January.

Penn also appears to hit out at the #MeToo movement in the book.

In the novel Honey writes a poem branding the movement - in which victims of sexual abuse have spoken out on social media - a ''an infantilising term of the day''.

The poem continues: ''Is this a toddlers' crusade? Reducing rape, slut-shaming and suffrage to reckless child's play? A platform for accusation impunity? Due process has lost its sheen?''

Penn originally claimed he had met Pariah at a writers' conference in Florida in 1979. The Oscar-winning actor did not admit he was the author of the audiobook until months later, when he revealed his plans to expand and publish the book.