Thousands of demonstrators are gathering in Madrid as the Spanish city hosts climate negotiations by the UN.
They are calling for more ambitious climate change policy.
The rally was joined by speakers including actor Javier Bardem and activist Greta Thunberg. A concert was also held near to Nuevos Ministerios, a government complex in the city centre.
Organisers say around 500,000 people are taking part in the demonstrations. Officials have not given a figure.
Simultaneous protests are also being held in the Chilean capital of Santiago, which was initially expected to host the UN conference.
"The change we need is not going to come from people in power," Ms Thunberg told the crowds. "The change is going to come from the people, the masses, demanding change."
The talks - known as the COP25 - were due to be held in Chile but the Chilean government cancelled following weeks of civil unrest.
They began on Monday with the UN secretary general warning that time to avoid the worst effects of climate change was running out and that negotiators should be guided by the science.
By the end of the meeting on 13 December, negotiators hope to resolve disagreements over the implementation of the Paris Climate Accords.
But countries continue to disagree on targets for cutting carbon emissions, and plans to increase these targets have not been included in the agenda for COP25's final agreement.
BBC (Photos: AFP)