Climate activist Greta Thunberg is facing a fresh wave of criticism after a recent speech and social media post were called out online.

The latest attacks on the teenager came as she began her journey home to Sweden, after spending months travelling the world calling for climate action.

Ms Thunberg posted a picture of herself sitting on the floor of a train, surrounded by luggage, on social media.

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She has been vocal about her decision not to travel on planes, which are considered to be contributing the climate change.

"Traveling on overcrowded trains through Germany. And I'm finally on my way home!" the picture was captioned.

The German railway company she was travelling with, Deutsche Bahn, was quick to weigh in on Ms Thunberg's post.

Deutsche Bahn responded to the teen's initial tweet, wishing her a safe journey home and adding that they were working hard on improving connections, trains and seats.

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However, the railway company later released a statement to media saying Ms Thunberg had a seat in first class between Kassel and Hamburg, and that other members of her team were sitting in first class from Frankfurt onwards.

Deutsche Bahn also tweeted twice more to comment on Ms Thunberg's train travels through Germany.

"Dear #Greta, thank you for supporting us railroad workers in the fight against climate change! We were pleased that you were on the ICE 74 with us on Saturday. And with 100 per cent green electricity," the company posted.

"It would have been even nicer if you also reported how friendly and competently you were looked after by our team at your seat in first class."

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Ms Thunberg later clarified that while she had initially been sitting on the floor, she later got a seat.

She also added that the fact she didn't initially sit in a seat wasn't meant as a knock against Deutsche Bahn.

"This is no problem of course and I never said it was. Overcrowded trains is a great sign because it means the demand for train travel is high!" she said.

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In the midst of that backlash, critics have accused Ms Thunberg of "inciting violence" in a speech to fellow young climate activists.

The 16-year-old was speaking at a climate protest in Turin, Italy, on Friday when the comments were made, met with cheers from the audience.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 39 seconds 39 s Greta Thunberg accused world leaders of 'running away from their responsibilities' as climate talks at COP25 end

In the speech, she pledged to put politicians "against the wall" when urging them to answer questions on their lack of climate action.

"World leaders are still trying to run away from their responsibilities, but we have to make sure that they cannot do that," she said.

"We will make sure that we put them against the wall and that they will have to do their job and to protect our futures."

Critics rushed to voice their disapproval of the teenager online.

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Ms Thunberg herself took to Twitter to apologise and explain the comments, explaining that the phrase was lost in translation.

"I said we must hold our leaders accountable and unfortunately said 'put them against the wall'. That's Swenglish: 'att ställa någon mot väggen' (to put someone against the wall) means to hold someone accountable," she posted.

"That's what happens when you improvise speeches in a second language. But of course, I apologise if anyone misunderstood this.

"I cannot enough express the fact that I — as well as the entire school strike movement — are against any possible form of violence. It goes without saying, but I say it anyway."

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Supporters of the young activist and the climate strike movement were quick to defend Ms Thunberg, saying that even by the English definition there were no violent undertones to her statements.

Social media is still abuzz with reactions to TIME magazine naming Ms Thunberg Person of the Year for her global movement urging world leaders to take faster action on climate change.

Ms Thunberg said she wanted to dedicate the award to all young activists who were involved in the so-called Fridays for Future movement, where students left school to protest a lack of climate action by governments.

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The 16-year-old became the youngest person ever to be named Person of the Year by the magazine, and the decision was met with both support and criticism.

The backlash was swift from critics and even came from world leaders, with US President Donald Trump again taking to Twitter to voice his disapproval of the magazine's choice.

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"Greta must work on her anger management problem, then go to a good old fashioned movie with a friend," Trump tweeted.

Ms Thunberg swiftly changed her Twitter bio to "a teenager working on her anger management problem. Currently chilling and watching a good old fashioned movie with a friend."

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AP/ABC