Extinction Rebellion activists Amberley Rankin and Jack Gray, both 16, are in Guildhall Square under the Big Screen, lying down with friends.
Their action is designed to draw attention to what Portsmouth College student Amberley said was leading chants demanding ‘climate justice’.
When a passer-by suggested the group should be at school, she shot back: ‘I would be at school but there’s a climate emergency.’
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Amberley told The News: ‘The future scares me and the adults aren't doing enough.
‘(We need) system change, not climate change.’
She added: ‘It's not one individual's fault, it’s the system.’
It comes as a day of action is being held worldwide on Valentine’s Day.
Fellow protester and Havant College student Jack added: ‘I'm involved because I don’t want our planet to die.
‘I want to feel like I’ve done something.’
The youngsters were in Guildhall with other protesters for the Climate Strike – with the government urged to do more.
Portsmouth South MP Stephen Morgan, who was at the strike, said: ‘It is evident government needs to listen to Portsmouth people when it comes to tackling the climate emergency.
‘Yesterday’s reshuffle saw a new environment secretary appointed (George Eustice), the fifth in five years, and the president of Cop26 climate (Claire O’Neill) conference ruthlessly sacked in the public eye.
‘What sort of message does this send about the government’s attitude to climate change? So far government has failed to deliver on 24 out of 25 emissions-reducing actions recommended by the climate committee, hit just seven of 24 indicators showing the UK’s progress towards its climate goals being met, and missed targets for the fourth and fifth carbon budgets.
‘Mr Johnson is an ambassador of indifference when it comes to the climate emergency with his own ministers’ publicly stating that he has “admitted he doesn’t really understand it”. The government needs to step up and give the climate crisis the leadership and urgency it deserves.’
Nick Sebley, of Extinction Rebellion, said: ‘The cabinet reshuffle just saw people moved into positions who supported Boris Johnson over Brexit not because they will do any good on climate change.
‘The new environment secretary has constantly voted against climate change mitigation and is just a “yes” man. He has a poor record on environmental issues.’