Thunberg arrived in Lisbon after nearly three weeks at sea, before giving a press conference at the port alongside youth activists and crew members.
“We need to work together to make sure that we secure future living conditions for humankind and that we fight for not only ourselves but for our children and for our grandchildren and for every single living being on earth,” the teenage activist told reporters.
“We will go to COP25 in Madrid and we will continue the fight there to make sure that within those walls, the voices of the people are being heard,” she added.
Around 25,000 people from 200 countries are expected to attend the COP25 climate change conference in Madrid this week. They include dozens of heads of state and government, business leaders, scientists and activists like Thunberg.
The Madrid meeting is the last gathering of the COP group before 2020 – the year when the Paris agreement comes into effect.
When nations signed the deal back in 2015, they agreed that global emissions of greenhouse gases must peak in 2020 at the latest, and then start coming down – or the world will face disastrous and irreversible damage.
CNN’s Hande Atay Alam and Ivana Kottasová contributed reporting.