Over 4,500 people planted some 20,000 trees in the Portuguese capital on Sunday.
The initiative was Lisbon's first as the European Green Capital 2020.
The city is aiming to be a hundred percent carbon-neutral by 2050 and aims to plant 100,000 trees throughout the year.
"Tree planting helps to counteract one of the most negative effects of climate change and global warming, which is the heatwaves," Lisbon Mayor Fernando Medina told reporters. "When these kinds of areas are planted, the surrounding temperature can drop by three to five degrees centigrade."
Apart from the title, Lisbon won a financial prize of 350,000 euros from the European Commission to kick-start its green capital year.
"Get organized as you have been but even more so that you force us the politicians to do the right things," European Commission Vice-President Frans Timmermans told the audience at the opening ceremony to launch the Lisbon initiative.
Time to end the war on nature
At the same ceremony the UN Secretary-General, Antonio Guterres, who is Portuguese, said it was time to get serious about the planet.
"This year, in 2020, we must provide the proof that we want to end the war that humanity has launched against nature," said Guterres.
Portugal's president, Marcelo Rebelo de Sousa, says his country is betting on green, betting on the environment and betting on facing climate change and global warming.
To that end, Lisbon will be launching environmental initiatives throughout 2020.