Scottish Greens co-leader Patrick Harvie has said he regrets the party isn't fielding a candidate in every seat in Scotland for the general election.

The Green MSP said his party's central message was a demand for "climate action" as he was interviewed on STV's Scotland Tonight.

He is the latest senior Scots politician to be interviewed on the programme by STV's political editor Colin Mackay as part of a series of election specials.

Harvie said the world had a "ten-year window of opportunity" to tackle the climate crisis - a period which could cover half of the period of office of the next UK Government.

He also defended his party's policy, unveiled in a general election manifesto on Monday, of free public transport across Scotland.

The Scottish Greens are fielding candidates in 22 of 59 seats north of the border.

Harvie told STV: "The world has been given a ten-year window of opportunity to take radical, transformational change and we know we can do that in a way that makes people's lives better and not worse.

"But the UK Government that's elected in December might, in theory, be in office for half of that ten-year window of opportunity.

"So they have to be given a serious challenge to demand climate action and to show that we can do that in a way that's fair and just."

Challenged on why - if tackling climate change is so pressing - the party is not fielding a full roster of Scottish candidates in this election, the co-leader said: "Westminster elections are a tough gig under the unfair voting system with First Past the Post for the Greens.

"For any small party, actually, it's pretty tough to able to stand everywhere."

Harvie added: "I do regret that we don't have a candidate in every seat."

He said he wouldn't advise voters which party to pick on environmental issues if the Greens are not on the ballot paper in their area.

But he called on people to ask the candidates in their seat where they stand on issues like continued fossil fuel extraction or Heathrow expansion, and if they are willing to challenge their own parties on such matters.

The party is calling for a "Green new deal" to help the country transition to a net-zero economy and to create what it claims will be hundreds of thousands of green jobs.

Asked if policies like free bus travel for under-21s and nationalising the energy grid are just "Labour-lite", Harvie said: "Very far from it."

He went on: "I welcome the fact that Labour are finally talking again in recent years about a boldly interventionist state.

"We need to rebuild the power of the public sector instead of pretending that the market can achieve everything."

A long-term aim to make all public transport free would be funded by raising vehicle excise duty on high-polluting cars and scrapping the fuel duty freeze, the Greens claim.

Pressed on how much cash would be generated by raising excise duty, Harvie answered: "I'm not going to remember every single figure for every single band.

"But we published that the other week and it would raise up to £500m."

You can watch the full interview with Patrick Harvie along with Scottish Brexit party candidate Sebastian Leslie at 10.40pm on Tuesday.

It is the latest in a series of election interviews with senior party figures to be broadcast on Scotland Tonight in the coming days.

STV will also host a general election debate featuring four of the Scottish party leaders on Tuesday, December 3.