Teenage activist Greta Thunberg said the world was underestimating "angry children" as she arrived for this year's international climate summit in Spain after a three-week journey crossing the Atlantic.

Her arrival came as the UN said this decade will be the warmest on record, with 2019 among the top three hottest years in history.

“Everyone should do what they can to be on the right side of history; no country is doing enough,” Ms Thunberg said after being welcomed at a Lisbon port by Mayor Fernando Medina.

She made the three-week journey on a catamaran with a British navigator and an Australian sailing couple after the summit was moved from Chile to Madrid because of unrest in its original host country.

Promising to continue her journey to the Spanish capital after a couple of days’ rest, Ms Thunberg said she would “fight so that the voices of the people and the young would be heard from the other side of the wall”.