Washington: US President Donald Trump has mocked Greta Thunberg, the Swedish 16-year-old climate activist, calling her distinction as Time magazine's Person of the Year "ridiculous" and suggesting she work on anger-management issues and go to a movie with a friend.

Trump's advice, in a morning tweet, came a day after Thunberg, who has mobilised millions of people to fight climate change and condemned leaders' inaction, became the youngest person to be dubbed Person of the Year by Time.

"So ridiculous," Trump tweeted. "Greta must work on her Anger Management problem, then go to a good old fashioned movie with a friend! Chill Greta, Chill!"

Trump was responding to a tweet congratulating Thunberg by Roma Downey, an actress and producer from Northern Ireland. Her production credits include the History Channel miniseries, The Bible, which she also starred in as Mary, mother of Jesus.