Thousands of students have walked out of class in Sydney and Melbourne as part of a nation-wide strike over inaction on climate change.

Friday’s country-wide mass protest comes months after students first skipped school over climate action, a move condemned by the Prime Minister.

This time students are taking part in a day of global action with more than 90 countries participating.

Thousands of school students from across Sydney attend the global #ClimateStrike rally at Town Hall in Sydney. Picture: AAPIMAGE

In Melbourne protesters gathered outside the the Old Treasury Building, blocking traffic on Spring Street.

In Sydney striking students plan to leave the Town Hall and march to Hyde Park.

A similar protest on Perth is planned for St Georges Cathedral at 11am.

The students have three demands: stop the Adani coal mine in central Queensland, no new coal or gas, and 100 per cent renewables by 2030.

More than 800 academics have signed an open letter in solidarity with the striking students, with some set to join in on protests on Friday.

Victorian high school student Milou Albrecht was central to organising the strikes and says she will be taking to the streets out of fear of not having a safe future.

Hundreds of thousands of students are expected to strike worldwide demanding urgent political action on climate change. Picture: AAPIMAGE

“We don’t want to strike,” the 14-year-old said.

“We love learning, that’s important for them to know.”

Milou and her friend Harriet O’Shea Carre were inspired by Swedish teen Greta Thunberg, who has been striking for climate action since last August.

Harriet hopes the prime minister calls a climate emergency, and says she will feel terrible if he again condemns their actions.

“It’s totally unfair considering they have been representing (voters) with the expectations they will do what’s in the best interests of the country,” she said.

Protesters at the #ClimateStrike rally in Sydney. Picture: AAPIMAGE

“They are letting us down every single day they don’t do that.”

Senior cabinet minister Christopher Pyne said the protests will damage the students education.

“Usually strikes are when employees withdraw their labour from an employee so I’m not sure why the students are withdrawing themselves from school. It only damages their education,” Mr Pyne told Nine’s Today on Friday.

He said the students should be in school, and if they wanted to engage in political activism it should be on their own time.

Labor frontbencher Joel Fitzgibbon defended student activism, saying children should be encouraged to express their opinions.

“I don’t think any student is going to miss out on their career because they missed a few hours of school today,” he said.

“As long as they are safe and their parents know where they are and they are marching or protesting in a respectful way, in a responsible way, I think that is fine.”