Scottish Greens co-convener Patrick Harvie has urged party members to campaign to rejoin the EU if Brexit goes ahead.
Speaking at the party conference in Greenock, he said the future of Scotland and the UK is European.
In a wide-ranging speech he said the Conservatives were treating the Scottish Parliament with "contempt", stressed the devolved powers row is an "assault on devolution" and urged reform of local taxation, saying the "outdated and unfair council tax must go".
He told the around 150 party members at the conference in the Beacon Arts Centre: "We know that our future, Scotland's, and the future of the other nations in these islands, are as European countries and that our future remains part of the European family.
"Over the course of today, we're asking you to join us in committing, not just to opposing the Brexit crisis now, not just to opposing the assault on devolution, but if this thing is done to us, if we are taken out of the European Union, let's commit now to campaigning to get back in because our future is European and we will be part of that project."
He said the reasons to oppose Brexit now were stronger than during the 2016 EU referendum, and include the Irish border issue, evidence of economic harm and threats to the devolution from the row around powers returning to the devolved administrations.
He said: "Theresa May's government cannot be trusted with these powers and we will not hand them over."
"We can have any Brexit we want as long is it the hardest of possible Brexits in the fever dreams of the empire 2.0 Brexit extremists," he added.
"We have the threat to our social, environmental and workplace standards from these same Brexit ultras who have been campaigning for years for a bonfire of the regulations."
Mr Harvie continued: "We don't just need to challenge the fundamental approach that the UK Government is taking to the Brexit process, we need to challenge the whole Brexit project itself."
Addressing council tax, he said: "With further attacks on Scotland's budget expected to come from the UK over coming years, we cannot simply allow the pressure to be pushed down the chain to local community level year after year.
"The reality of this is that council tax, finally, has to go. How many elections did the SNP fight saying that? It's about time to deliver.
"We need new local fiscal freedoms and new opportunity for councils to make choices that are right for their circumstances."
Mr Harvie said the party would also push for a "proper ban" on fox hunting, campaign for net zero carbon emissions by 2020 and "lead opposition to the SNP's unwanted and unnecessary education reforms".
Leader of the Northern Ireland Greens, Steven Agnew MLA, also spoke, saying remaining in the single market and customs union is the only solution to the Irish border issue.
He warned any physical border would lead to "increase the likelihood of violence" and compared a "mythical" technical solution to a unicorn.