Climate campaigner Greta Thunberg was fined on Monday for disobeying Swedish police at a rally last month, but said she acted out of necessity due to global warming and joined a new protest hours later.

The 20-year-old activist, who has become a key face of the movement to fight climate change, was accused of disrupting traffic and refusing to leave a June protest in port city Malmo.

"It's correct that I was at that place on that day, and it's correct that I received an order that I didn't listen to, but I want to deny the crime," Thunberg told the court when asked about the charge against her. 

Thunberg said she had acted out of necessity, citing the need created by the "climate crisis".

The rally, organised by environmental activist group Reclaim the Future, tried to block the entrance and exit to the Malmo harbour to protest against the use of fossil fuel. 

"According to me we are in an emergency, and then due to that my action was legitimate," she told reporters after the trial.

After a short trial, the court nonetheless found her liable and issued a fine of 1,500 kronor ($144) plus an additional 1,000 kronor to the Swedish fund for victims of crime. 

The crime she was convicted of can carry a maximum sentence of six months in prison, but usually these types of charges result in fines.

- Not backing down -

Asked if she would exercise more caution in the future following her fine, Thunberg said they would "definitely not going to back down".

"We know that we cannot save the world by playing by the rules because the laws have to be changed," the activist said.

"It is absurd that the ones acting in line with the science, the ones blocking the fossil fuel industry are the ones having to pay the price for it," she added.

Hours later, Thunberg joined a protest similar to the one in June that resulted her being fined.

Sitting on the road leading to the Malmo port she put out a sign reading "I block tanker trucks".

Thunberg shot to global fame after starting her "School Strike for the Climate" in front of Sweden's parliament in Stockholm at the age of 15.

She and a small band of youths founded the Fridays for Future movement, which quickly became a global phenomenon.

In addition to her climate strikes, the young activist regularly lambasts governments and politicians for not properly addressing climate issues. 

Reclaim the Future insists that despite the legal pressures, it remains unbowed in its determination to stand up to the fossil fuels industry.

"If the court chooses to see our action as a crime it may do so, but we know we have the right to live and the fossil fuels industry stands in the way of that," the group spokeswoman Irma Kjellstrom told AFP.

Six people present at the June protest have or will face charges at the court in Malmo.

"We young people are not going to wait but will do what we can to stop this industry which is burning our lives," she said, explaining the group's plans for continuing civil disobedience.

bur-jll/jmm