Nicola Sturgeon says she may "accelerate" plans for a second referendum on Scottish independence as she expressed "profound concern" Boris Johnson becoming Prime Minister.
But pro-union politicians in Scotland called on the new leader to reject calls for another referendum and focus on keeping the UK together.
Johnson was also accused of representing a "dangerous form of English nationalism" after defeating Jeremy Hunt today.
The Scottish Government has legislation gong through Holyrood which would pave the way the for a second referendum on leaving the UK. Senior nationalists joined in the calls for another vote, as Ms Sturgeon indicated she may step up her plans.
"We will consider whether the timetable we've set out to have it on the statute book by the end of this year is still the right one or whether we should accelerate that," Ms Sturgeon told STV News.
"Then, of course, we will move forwards on that basis. "But my message to Boris Johnson, and to any politician who takes the undemocratic stance of thinking they can block the will of the Scottish people, is simple.
"You can oppose independence, that is absolutely legitimate, but what is not legitimate is to try to stand in the way of the people of Scotland having the right to choose."
The First Minister had earlier I congratulated Mr Johnson on his election as Tory leader but said she has "profound concerns" about the prospect of his premiership.
"These are concerns that I am certain will be shared by the vast majority of people in Scotland who, had they been given any say, would not have chosen to hand the keys of Number 10 to someone with his views and track record," she added.
“Mr Johnson should be in no doubt about the gravity of the situation he is about to inherit as Prime Minister nor, in particular, about the strength with which I and others will oppose his threat of a No-deal Brexit.
“Brexit of any kind would be deeply damaging to Scotland and the rest of the UK, but his public pledge to leave the EU by October 31st – ‘come what may’ and ‘do or die’ – flies in the face of logic, common sense or any basic regard for the well-being of the people and nations of the UK.
“It is a deeply irresponsible threat, and not one that should be contemplated by any serious political leader. It should now be taken off the table without delay or equivocation."
The Scottish Government has warned that No-deal Brexit would damage employment north of the border, investment and living standards as well as posing "serious and totally unnecessary questions" on critical issues such as medical supplies.
“The Scottish Government will be at the forefront of all and any moves to stop Brexit and block a No-deal Brexit, as will SNP MPs in the House of Commons," Ms Sturgeon added.
“We will be joined in those efforts by others from across the political spectrum – and the fact that Mr Johnson already faces strong opposition from members of his own party who oppose a No-deal Brexit underlines the seriousness of the position he finds himself in.
“Scotland did not vote for Brexit, or for the current Tory Government – and certainly not for Boris Johnson as Prime Minister. All of this underlines the need for Scotland to have the right to determine our own future, in line with the democratic wishes of all those who live here.”
Labour leader Richard Leonard also hit out at Johnson's election.
"Boris Johnson represents a dangerous form of English nationalism, and the one certainty of his election as leader of the Tory party is more uncertainty for the future of Great Britain and Northern Ireland," Leonard said.
“Johnson some time ago abandoned the unionist tradition of the Conservative and Unionist party. The Tory Party he now leads is a real and present danger to Scotland’s place in the UK.
“A no-deal Brexit led by Boris Johnson spells disaster for our country, and it is unforgivable for the Scottish Tories to back him.
“Only Scottish Labour is standing up for Scotland’s place in the UK, for the UK’s place in Europe and against a harmful no deal Brexit.”
Greens co-leader Patrick Harvie said Scots must now be given the chance to "escape Boris Johnson’s Brexit Britain" after is victory over Jeremy Hunt.
Harvie said:"“The people of Scotland didn’t vote for Brexit, we didn’t vote for a Tory government and I’m certain that given the choice we wouldn’t have voted for Boris Johnson either.
“Scotland needs a route out of Boris’ Brexit Britain, and while we already have a firm mandate to hold a referendum, Johnson’s elevation to the office of Prime Minister on the back of bluff and bluster is hugely concerning and reinforces the need to hold this vote urgently.
“Brexit is already having a hugely detrimental impact with EU citizens in particular feeling under attack as rhetoric from the likes of Johnson and co has been relentless over the last three years. In the face of this utter shambles it has never been more important that Scotland retakes our place as an independent European nation.”
But Pamela Nash, chief executive of Scotland in Union, called on the new Tory leader to protect the union.
“Boris Johnson must prioritise keeping the UK united, which means making the country work for all of its citizens and ensuring the people of Scotland want to remain part of the UK," she said.
“Predictably, Nicola Sturgeon and the SNP are already using his victory to launch yet another attempt to divide us, and the new Prime Minister must not rise to the Nationalists’ desperate grievance tactics or stoop to their level. We are stronger together as part of the UK.
“Whatever your views on Boris Johnson, the very last thing Scotland needs is another divisive independence referendum and more constitutional chaos. Let’s hope that Mr Johnson becomes the Prime Minister the country needs at this vital time.”
And newly elected Liberal Democrat Leader Jo Swinson added: "Boris Johnson has finally got his hands on the keys to Number 10, but he has shown time and time again that he isn’t fit to be the Prime Minister of our country.
"Whether it is throwing people under the bus or writing a lie on the side of one: Britain deserves better than Boris Johnson.
"If we want to defeat nationalism and populism, we need to give people an alternative vision for our country. It’s the Liberal Democrats who can lead the renewal our country needs."