The Green co-leader Jonathan Bartley says the Home Office should be broken up in response to failures over the hostile environment policy, as he seeks to position the party as “the antidote to populist authoritarianism” from Boris Johnson and others.

The Greens are holding their annual conference in Newport following success in local and European elections. Bartley’s speech to the event calls for radical change to tackle the anxieties of voters, exemplified by the Brexit vote.

One idea is to split the Home Office into a ministry of the interior, centring on law and order, and a department focused on a more liberal approach to immigration and refugees, called the ministry of sanctuary.

In an interview with the Guardian before his speech, Bartley said failures such as the Windrush scandal had epitomised the Home Office’s structural problems, illustrating a wider political crisis that left many people, especially younger Britons, feeling “shut out of power”.

“There’s a growing realisation that the Home Office isn’t fit for purpose in so many ways – it’s the hostile environment, the debacle over detention centres. There’s a culture there that is perverse and is dangerous,” he said.

“I think there’s a growing realisation that something has to change. Whether anyone will be bold enough, I’m not sure.”

Green proposals on a more open migration system, a much-reduced prison population and the closure of immigration detentions centres were a stark contrast to the increasing populist political message from Johnson, Bartley said.

“When you’re calling judges the enemy of the people, and you’re manipulating the constitution, and unrepentant when you’re caught out – this is very, very dangerous,” he said. “It’s what we’re seeing right across the continent, and coming now into the Conservative party.”

While their poll ratings are currently about 5%, the Greens are hopeful of performing better in a likely imminent general election after gaining almost 200 councillors in May’s local elections, and then seven MEPs at the European vote, and recently passing 50,000 members.

The party’s core message on tackling the climate emergency had been buoyed, Bartley said, by the focus on the Swedish teenage activist Greta Thunberg and Extinction Rebellion, who are launching a new wave of protests next week.

“We feel like we’re the political expression of the school strikes movement and the Extinction Rebellion movements,” Bartley said.

“And the big change has been the focus on systems change – recognising that it’s not going to be done just by more recycling or taking fewer flights. Government has to set the conditions for this to happen.”

At its conference last month, Labour voted to embrace a so-called green new deal, aiming to reshape the economy around sustainable practices – a longstanding Green idea.

Bartley said his party’s version was notably less centrist than Labour’s, with a “bottom-up” approach including providing £10bn a year from higher corporation tax to local authorities, which would get new powers to spend it on renewable energy and local transport.

At Westminster, a “carbon chancellor” would introduce policies such as free bus transport, and all petrol and diesel cars being phased out by 2030.

“Every decision in government would be made through the lens of both the climate emergency and people’s wellbeing, challenging that blunt instrument of GDP, which was supposed to solve everything,” Bartley said.

The old economic paradigm was “increasingly outdated”, he argued, with people having been falsely sold a vision several decades ago about greater wealth and a technological revolution improving their lives.

“The economy is now three times bigger – we’re three times wealthier as a country,” Bartley said. “We’ve had the technological revolution, we’ve discovered how to harness the wind and the sun and the tides.

“But we’ve got 1.4m emergency food bank parcels every year, we’ve got a mental health crisis, we’re on the brink of environmental apocalypse. And we’ve got a generation which, for the first time, is going to be worse off than their forebears. So the system isn’t working.

“The response of the Conservatives and others is this populist authoritarianism, to scapegoat, divide and rule.”