Created by the World Wildlife Foundation (WWF) Earth Hour starts at 8.30pm and lasts until 9.30pm. It aims to raise awareness about climate change and the loss of biodiversity.
The initiative is being replicated in many countries around the world as a symbolic action to call attention to the need for people to change their behaviour and defend the planet.
Some of the monuments in Portugal that have agreed to douse their lights are Cristo Rei, São Jorge castle, Belém tower and Jerónimos Monastery in Lisbon, the Sea Museum in Cascais, castles in Guimarães, Pombal, Silves and Figueira de Castelo Rodrigo and the D. Luiz bridge in Porto.
The number of Portuguese municipalities joining this 11th edition of the Earth Hour is lower than that 144 in the 2017 edition, the largest number ever in Portugal.
Last year, Earth hour broke the world record with 3,100 monuments going dark in 187 countries and territories between 8.30pm and 9.30pm.