The co-leader of the Green Party has called on “progressives” in Parliament to challenge “brutal” Tory policies, having held her Brighton Pavilion seat.
Caroline Lucas accused Theresa May of running an “arrogant and insulting” campaign focused on “extreme Brexit”.
With one constituency undeclared, the prime minister says she will work with the DUP to form a government after failing to secure a majority.
Green co-leader Jonathan Bartley said it would be “a coalition of chaos”.
Ms Lucas took the Green’s one seat in the election, out of 457 candidates.
She said: “What is very clear is that Theresa May did not get the bigger mandate she was asking for, for the kind of extreme Brexit that she has been pursuing.
“I’m hoping very much the progressives across Parliament will work together to challenge that kind of a Brexit which is brutal, damaging and wrong.”
The Tories are forecast to be the biggest party with 319 seats out of 650, ahead of Labour on 261, the SNP on 35 and the Lib Dems on 12. The DUP won 10 seats.
Mr Bartley, who did not stand in the election, said it was “extremely worrying” that the DUP could be “holding sway” over the government.
“The DUP I don’t think are the kind of people you want calling the shots,” he told BBC News.
“Now what’s going to happen with the DUP and their climate change scepticism,” he added. “Are they going to exact a very serious price for the Government being propped up in this way?”
Throughout the campaign, the Greens have called for a “progressive alliance”, writing to Labour and the Liberal Democrats in an attempt to do deals and oust the Tories.
Although the parties shunned the idea nationally, some local activists have supported it and saw candidates step aside for one another.
In Ms Lucas’s own seat of Brighton Pavilion, the Lib Dems stepped aside, while the Greens decided not to contest Labour-held Ealing Central and Acton, and Brighton Kemptown.
Some prominent figures have also backed the plan, including Lib Dem Sir Vince Cable, who was re-elected in Twickenham.
But after the exit poll was published, former Lib Dem leader Sir Menzies Campbell told the BBC that the difference in approach to Brexit between the Lib Dems and Labour would stop any progressive alliance being formed.
Ms Lucas had tweeted on Thursday night – when exit polls indicated a disappointing result for the Conservatives – “hardly dare hope this is right. To be clear, Greens will *never* support a Tory government #HopeoverHate”.
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