Nicola Sturgeon has urged voters across Scotland to support the SNP in next week’s EU elections “whether you’re for or against independence”, to send a convincing message about the country’s opposition to Brexit.

While opponents have accused her of using the Brexit deadlock as an excuse to campaign for independence, Sturgeon stepped up her appeals to pro-European voters to discount Labour.

Launching her party’s EU manifesto on Friday, she dismissed as “pointless” Jeremy Corbyn’s talks with Theresa May, and accused him of wanting to overturn the referendum result in Scotland, where a majority voted to remain.

Speaking to an audience of candidates and activists hours before the Brexit party leader, Nigel Farage, was due to hold a rally in Edinburgh, Sturgeon said: “We have senior Tories now openly calling for an electoral pact with Nigel Farage … it is a deadly serious possibility and for Scotland it would be a nightmare.

“The future of our country is at stake and people in Scotland deserve the right to decide whether Scotland should become an independent member of the EU. Whatever your views on independence and whether you voted remain or leave in 2016 one thing is clear and beyond doubt. Westminster right now is failing all of us.”

Describing the manifesto as setting out “a positive, progressive European future for our nation”, she pledged that SNP MEPs would argue for a Europe-wide green new deal and press for the development of the internal energy market to boost renewable energy projects.

She said the manifesto underlined the party’s “strong and unequivocal” support for free movement, and promised to push for reform of the common fisheries policy to suit “different fleets and different geographies”.

Earlier, Sturgeon said the prospect of Boris Johnson becoming prime minister would “horrify” Scots and boost the case for independence.

The former foreign secretary was a “complete and utter charlatan”, Sturgeon told BBC Radio 4’s Today programme.

Speaking a day after Johnson confirmed his bid to become Conservative leader, Sturgeon said: “The prospect of him becoming prime minister is one that will horrify many people across Scotland.”

Ruth Davidson, the leader of the Scottish Conservatives, frequently criticised Johnson during the EU referendum, and blocked him from attending a conference of the Scottish Tories earlier this month.

Alluding to the rift, Sturgeon said: “Even the Scottish Tories are terrified at the prospect [of a Johnson premiership] because I think they know what it would do to their standing.”

She added: “If we wait too long [for another vote on independence], then we risk untold damage being done to Scotland’s interests, certainly by Brexit, and then possibly the prospect of having somebody like Boris Johnson as prime minister.”

Asked what impact a Johnson premiership would have on the independence debate, she said: “It would lead to many more people thinking that the best future for Scotland is to be independent so that we can protect our interests, that we can take our own decisions, that we can cooperate with other independent countries across the European Union for our mutual interest. In other words, be a normal, progressive, independent country.”

She described Johnson as “the guy who misled people in the Brexit vote, who has only ever put his own interests first … a complete and utter charlatan in my opinion”.

Sturgeon also said she could “feel a degree of sympathy” for Theresa May on a personal level, but added: “That said, I don’t think she has played the hand she was given particularly well.”

Sturgeon said of the prime minister: “I think she’s taken a lot of decisions that nobody made her take that have made the situation a lot worse.”