The demonstration, organised by the Fridays For Future movement, was part of a series of world-wide protests that were expected to be confined to small gatherings and online activism due to the COVID-19 pandemic.
The young activists were calling for immediate climate action, holding banners reading "Stop Denying the Climate is Dying" and "Don't Be a Fossil Fool."
Many had slogans on their masks.
Thunberg said the aim of the protests was to raise awareness, impact public opinion, and put pressure on people in power to take action to solve the climate crisis.
After taking part in the protest, Thunberg made her way to school, riding on her bike.
The world-wide student strike movement started in August 2018, when the 16-year-old Thunberg began protesting outside the Swedish parliament on school days. She has since been nominated for the Nobel Peace Prize.
GRETA THUNBERG: Well, the main hope is as always to try to affect to have an impact on the level of awareness and the public opinion, so that people will start becoming more aware of the climate crisis. So that we build up this pressure on the people in power, so that's, yeah, something happens.
And If we are not allowed to be more than 50 people, then we won't be more than 50 people. That we will be fewer people, and we'll spread out. We are striking on a few hundred places in Sweden today, so we focus on being few people in many places and stuff and keeping distance.
We need to treat the climate crisis as this crisis. It's just as simple as that. The climate crisis has never once been treated as a crisis. And unless we treat it as a crisis, we won't be able to so-call solve it.
- Would you like to see more scientists in the media? Would you like to see more activists in the media? What would you like to see?
GRETA THUNBERG: Just telling the truth as it is.