Former United States of America First Lady Michelle Obama has come to the defence of Swedish environmental activist Greta Thunberg after she was attacked by President Donald Trump.

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In a warm tweet, Mrs Obama encouraged the teenager to keep doing her good work as there are people who are behind her.

“Greta Thunberg, don’t let anyone dim your light. Like the girls I’ve met in Vietnam and all over the world, you have so much to offer us all. Ignore the doubters and know that millions of people are cheering you on,” the former first lady encouraged the climate activists.

The US president had earlier on mounted an attack on Greta after she was announced as this year's Time Person of the Year, an announcement that Trump described as ridiculous.

“So ridiculous,” Mr Trump said. 

“Greta must work on her Anger Management problem, then go to a good old fashioned movie with a friend! Chill Greta, Chill!” he added.

Thunberg herself has never been a fan of the unconventional president of the US, who does not share her concerns about the dangers that climate change poses to the planet.

Trump announced this year the withdrawal of the US from the Paris Climate Accord, saying that it puts his country at a great disadvantage.

Thunberg has not yet responded to the former first lady's warm message according to the People Magazine.