The Labour leader and Scottish First Minister joined a delegation including shadow chancellor John McDonnell and shadow Brexit secretary Sir Keir Starmer to discuss blocking a "blind" separation from the EU.
They met the leader of the Liberal Democrats, Sir Vince Cable, and other senior representatives from the Greens and Plaid Cymru.
It comes as a series of key Labour MPs - who represent Leave-backing constituencies that the government was relying on to help pass a Brexit deal - revealed they would be voting against it.
Speaking to Sky News after the opposition parties' summit, Mrs Sturgeon said there was a united anger at the government's deal.
She said they refused to accept the "spin" that MPs could trigger a no deal Brexit by rejecting the agreement expected to be signed off in Brussels on Sunday.
"The next part of this process, and arguably the most important part, is to build a consensus around the alternative," Mrs Sturgeon said.
"We know what the options are - the single market and the customs union, a People's Vote are two of those options.
"Extending A50 (Article 50) almost becomes an inevitably if we're going to leave the room open for these other options." [...]
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