The SNP has won a vote seeking permission to hold a second referendum on independence.
However, the decision is not legally binding and any new ballot would need to be approved by the UK Government.
MSPs voted 69 to 59 to mandate the First Minister to seek permission from the UK Government for a ballot to be held.
Nicola Sturgeon's minority Scottish Government won the vote following an extended debate thanks to support from the Scottish Greens.
- ITV News Scotland Correspondent Peter Smith reports from Edinburgh
The two-day debate started last week but was suspended on Wednesday as news of the terror attack at Westminster emerged.
Ms Sturgeon said she would seek to negotiate with the British government "in good faith and with a willingness to compromise" over the terms of a new referendum.
Should that fail, she promised to inform the parliament of next steps after its Easter break next month.
Ms Sturgeon said: "It is now the will of Scotland's democratically-elected national Parliament that discussions should begin with the UK Government to enable an independence referendum to be held.
"Today's vote must now be respected. The mandate for a referendum is beyond question, and it would be democratically indefensible - and utterly unsustainable - to attempt to stand in the way of it.
"We will now act on the mandate given to us by Parliament by making a formal approach to the UK Government within the next few days, after Article 50 has been triggered.
"This is, first and foremost, about giving the people of Scotland a choice on this country's future.
"The Prime Minister says that now is not the time for a referendum. I agree with that, which is why I have indicated a timescale no earlier than 18 months from now, when the terms of Brexit are clear - something the PM has now indicated she agrees with.
"It is up to the UK Government to now make clear when they consider a referendum would be appropriate."
Voters rejected independence in a 2014 referendum that was branded 'once-in-a-generation' by the SNP - but Ms Sturgeon says Brexit has changed the situation dramatically.
A UK Government spokeswoman said: "The Prime Minister has been clear that now is not the time for an independence referendum, and we will not be entering into negotiations on the Scottish Government's proposal.
"At this point, all our focus should be on our negotiations with the European Union, making sure we get the right deal for the whole of the UK.
"It would be unfair to the people of Scotland to ask them to make a crucial decision without the necessary information about our future relationship with Europe, or what an independent Scotland would look like.
"We have been joined together as one country for more than 300 years. We've worked together, we've prospered together, we've fought wars together, and we have a bright future.
"At this crucial time we should be working together, not pulling apart."
The First Minister wants to hold a new ballot between autumn 2018 and spring 2019, when she says the UK's deal with the European Union will become clear.
The Prime Minister Theresa May said yesterday after meeting Ms Sturgeon in Glasgow that a vote during that time frame would be "unfair" to the Scottish people.
"My position is very simple and it hasn't changed," she said. "It is that now is not the time to be talking about a second independence referendum and that's for a couple of reasons.
"First of all, now is the point when we are triggering Article 50, we're starting negotiations for leaving the European Union.
"Now is the time when we should be pulling together, not hanging apart. Pulling together to make sure we get the best possible deal for the whole of the UK.
"Also I think it would be unfair on the people of Scotland to ask them to make a significant decision until all the facts were known, at a point where nobody knows what the situation is going to be.
"My position isn't going to change, which is that now is not the time to be talking about a second independence referendum.
It follows a series of talks between UK ministers and those from the devolved nations over the UK's approach to leaving the EU.
Scottish ministers say there has been no clarity over how Scotland's interests will be represented as the Brexit process gets under way, and the role the Scottish Government will play in negotiations.