Former First Lady Michelle Obama has told climate activist Greta Thunberg to 'ignore the doubters' and to stand firm in her beliefs after receiving mocking tweets from President Trump.

'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,' Mrs Obama wrote on Friday morning.

Obama recently traveled to Vietnam with actress Julia Roberts as part of the Obama Foundation's Girls Opportunity Alliance program which looks to empower adolescent girls around the world through education.

Michelle Obama sent Greta Thunberg a public message of support on Twitter. She is pictured here alongside actress Julia Roberts in Vietnam where they were both on a visit to promote girls' education

Michelle Obama tweeted a message of support to Greta Thunberg just a day after President Trump slammed the climate change activist

After the President tweeted, Thunberg updated her Twitter bio to reflect Trump's comments.

Her message of solidarity came in response to a vitriolic attack on the teenager by President Donald Trump who tweeted on Thursday that he was not happy at the decision by Time magazine to name her Person of the Year for 2019.

'So ridiculous,' the Commander-in-chief wrote. 'Greta must work on her Anger Management problem, then go to a good old fashioned movie with a friend! Chill Greta, Chill!'

After the President tweeted, Thunberg updated her Twitter bio to reflect Trump's comments.

'A teenager working on her anger management problem. Currently chilling and watching a good old fashioned movie with a friend.' it read.

Her Twitter biography normally reads: '16-year-old climate and environmental activist with Asperger's.'

Time said it chose to honor the 16-year-old 'for sounding the alarm about humanity's predatory relationship with the only home we have, for showing us all what it might look like when a new generation leads.'

Trump said on Thursday it was 'so ridiculous' that the 16-year-old was named Time magazine's Person of the Year

In September, after Thunberg gave a passionate speech to the UN about climate inaction, Trump tweeted: 'She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!'

Thunberg altered her Twitter bio shortly after. It read: 'A very happy young girl looking forward to a bright and wonderful future.'

Earlier in the week, Thunberg was called a 'little brat' by Brazilian President Jair Bolsonaro.

In response, Thunberg changed her Twitter bio to 'pirralho' which means 'brat' in Portuguese.

Many praised the teenager for outmaneuvering Trump on Twitter.

By Thursday lunchtime the hashtag #GretaOutdidTrump was trending with one popular response reading: 'A 16-year old with Asperger's and non-native English speaker has better social and communications skills than the current President of the United States. And better Twitter game.'

Another wrote: 'Interesting how a leader that acts like a child is yelling at a child who acts like a leader'.

Swedish environment activist Greta Thunberg gave a speech at the COP25 Climate Conference in Madrid, Spain on Wednesday

Greta Thunberg glared at Trump when the two crossed paths with one another at the United Nations in September

Time said it chose to honor the 16-year-old 'for sounding the alarm about humanity's predatory relationship with the only home we have, for bringing to a fragmented world a voice that transcends backgrounds and borders, for showing us all what it might look like when a new generation leads.'

Democratic Party members rallied behind Thunberg, who has Asperger syndrome, highlighting the irony of Trump insulting a child on the internet when his wife Melania runs an anti-bullying charity.

Senator Bob Casey, a Democrat from Pennsylvania, criticized Mr. Trump for his 'decision to mock a child – AGAIN,' saying it is 'beneath the office of the President and an insult to people throughout the world.'

Democratic presidential frontrunner Joe Biden, also chastised Trump for his comments, writing on Twitter, 'What kind of president bullies a teenager?'

Thunberg's accusations that leaders are failing younger generations have made headlines, including her shouts of 'How dare you?' at the U.N. General Assembly

'Greta must work on her Anger Management problem, then go to a good old fashioned movie with a friend! Chill Greta, Chill!' President Trump wrote.

On Thursday Meghan McCain said she didn't believe Thunberg, 'earned' the honor of "Person of the Year' during a segment on The View.

While discussing the announcement with her talk show colleagues, the conservative co-host said she doesn't put much stalk into Time's long-standing award.

'I don't put a lot of clout into the [Time Magazine's] Person of the Year. Keep in mind, Hitler and the Ayatollah were also the Person of the Year,' she said.

McCain said that she wanted to see the whistleblower receive the title, noting that it would have been a 'more salient and timely choice.'

While admitting that she disagrees with Thunberg's opinions and movement, McCain says she finds it difficult to talk about the teenaged eco-warrior because of her age.

She said: 'I still don't believe in attacking underaged people. I disagree with her, I think everyone knows how I feel about climate change as religion on the left, but I'm not going to attack a 16-year-old in the same way I don't agree with attacking Barron Trump.'

'I don’t agree with her as a choice, and I also think if you are 16, is this peak for her? What comes next? I mean, this is a high, high bar for her,' she continued.

'I don’t know how you can possibly live up when you are 17 when you’re already person of the year.'

Meghan McCain (pictured) said she doesn't believe eco-warrior Greta Thunberg 'earned' Time's Person of the Year

Don Jr., the president's first son, piggybacked off his father's remarks and took aim at the teenager from his own Twitter account.

'Time leaves out the Hong Kong Protesters fighting for their lives and freedoms to push a teen being used as a marketing gimmick. How dare you?' he wrote.

In a separate tweet, Don Jr. shared a doctored photo of Time Magazine's Person of the Year cover with Trump's face replacing Thunberg.

He said: 'When it comes to keeping his promises, there's only one Person Of The Year: 'Booming Economy, Record Job Creation, Historic Tax Cuts, [America First] Trade Deals, ISIS Destroyed, Building the Wall.'

Don Jr.: ''Time leaves out the Hong Kong Protesters fighting for their lives and freedoms to push a teen being used as a marketing gimmick. How dare you?'