Greta Thunberg has been fined for disobeying a police order to leave a climate protest in the Swedish port of Malmö last month.
The climate activist, 20, denied the charge. "My actions are justifiable," she said during a brief hearing.
The court in Malmö however found her guilty and fined her 2,500 Swedish krona ($240; £187), AFP reports.
Thunberg was part of a group of protesters that blocked the road for oil trucks in Malmö harbour.
She was charged for failing to leave when ordered to do so by police.
"I believe that we are in an emergency that threatens life, health and property," she told the court, adding that "countless people" were at risk.
"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," she said.
The rally in June, organised by environmental activist group Ta tillbaka framtiden (Reclaim the Future), tried to block the entrance and exit to the Malmö harbour to protest against the use of fossil fuels.
"We choose to not be bystanders, and instead physically stop the fossil fuel infrastructure. We are reclaiming the future," Thunberg said in an Instagram post at the time.
Reclaim the Future says that it remains determined to stand up to the 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," group spokesperson Irma Kjellstrom told the AFP news agency.
Greenhouse gases have been pumped into the atmosphere by activities such as burning fossil fuels. The world has warmed by about 1.1C than it was in the late 19th century.
Climate activists have targeted the fossil fuels industry. Much of the oil and gas industry says that continued production is necessary in order to meet global energy demands.