MADRID Swedish environmental activist Greta Thunberg helped lead tens of thousands of protesters advocating for global action to combat climate change Friday evening just a few miles from where world leaders are convening for the United Nations climate summit.

Demonstrators from dozens of countries marched down Paseo del Prado in central Madrid, drumming and chanting. Among the signs in the crowd: Denial is suicide, Dont be fossil fools, and a giant banner reading No hay planeta B (There is no planet B).

Organizers estimate the event drew more than half a million people, although no official tally was available late Friday.

Thunberg arrived in Madrid by train on Friday after sailing by catamaran across the Atlantic Ocean from the United States to Lisbon, Portugal all part of an effort to use carbon-free transportation.

The 16-year-old earned celebrity status after she went on strike from school last year following Swedens hottest summer on record. For weeks, she sat outside her countrys Parliament, holding a School strike for climate sign and calling for her home country to enact policies in line with the goals of the 2015 Paris agreement on climate action. Her actions inspired thousands of other school strikes, and in March an estimated 1.4 million young people in more than 100 countries mobilized for a global strike , part of what has come to be known as the Fridays for Future movement.

Earlier Friday, Thunberg joined other teen activists for a staged sit-in at the U.N. 25th Conference of the Parties. At a press conference ahead of Fridays demonstration, Thunberg stressed that the world cant afford to delay action any longer.

We have been striking for over a year and still basically nothing has happened, she said. The climate crisis is still being ignored by those in power. We cannot go on like this. It is not a sustainable solution that children skip school.

Thunberg also called on members of the media to not just focus on her.

Im just activist, a climate activist, she said. A small part of a very big movement. And, yes, we need more climate activists.