In a video message, former ABBA member Bjorn Ulvaeus hit out at the people criticising fellow Swede, 16-year-old climate activist Greta Thunberg.
Mr Ulvaeus, who was a member of the Swedish supergroup, called Ms Thunberg, who started her school strikes outside the Swedish parliament last August, a girl with "superpowers" and a "populist nightmare".
He said he could not understand where the anger directed toward Ms Thunberg comes from.
"How can you stoop so low as to criticise the looks of a child? A young girl. The reason is of course the lack of counter-arguments," he said.
Ms Thunberg's outspokenness and confrontational approach - factors that have made her the global face of climate activism - have also gained her detractors, as has her shaming of those who travel by plane.
The likes of US President Donald Trump and Russian President Vladimir Putin have both taken aim at Ms Thunberg.
Mr Putin has said he did not share the excitement about Thunberg's address to the United Nations last month and Mr Trump took to Twitter to comment on the speech in which she angrily denounced world leaders for failing to tackle climate change.
Mr Trump said: "She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see."
"Even if you don't think that everything Greta says or does is unquestionable, you have to admire her guts and her focus. It's a monumental achievement to create a global mass movement from scratch in such a short time," Mr Ulvaeus said.
Inspired by Ms Thunberg's solitary weekly protests outside the parliament, millions of people have poured onto the streets around the globe to demand governments take emergency action on climate change.
ABBA is one of the most commercially successful groups in history with more than 375 million albums and singles sold. They released nine studio albums between 1973 and their break-up in 1982.