Some 160 MPs have joined a cross-party plan to stop a no-deal Brexit being forced through, by "using whatever mechanism possible".
The plan aims to stop Prime Minister Boris Johnson forcing through no deal by shutting down Parliament and will see MPs "do whatever is necessary" to stop that happening.
Senior opposition politicians including shadow chancellor John McDonnell, Liberal Democrat leader Jo Swinson, SNP Westminster leader Ian Blackford, Plaid Cymru Commons leader Liz Saville Roberts, Green Party MP Caroline Lucas and Change UK leader Anna Soubry were at the meeting.
Mr McDonnell said the PM was a "threat to democracy" through not ruling out proroguing Parliament to push through a no-deal Brexit.
Labour MP Stephen Doughty said the declaration has "160 signatures" from MPs and he invited other politicians of all parties to join them in signing the document.
It came after opposition party leaders, including Jeremy Corbyn - who was unable to attend the Church House meeting due to a diary clash - held a productive meeting aimed at stopping no deal Brexit.
The group of MPs agreed "on the urgency to act together to find practical ways to prevent no deal, it said in a joint statement, "including the possibility of passing legislation and a vote of no confidence".
The legislative route to stopping no deal was preferred over the alternative, MPs in the meeting signalled, before heading to Church House for the day's second meeting.
Meeting at Church House in Westminster - the meeting place of the Houses of Parliament during the Second World War - the MPs signed what they are calling the 'Church House Declaration'.
The text of the Church House Declaration reads: "Shutting down Parliament would be an undemocratic outrage at such a crucial moment for our country, and a historic constitutional crisis.
"Any attempt to prevent Parliament sitting, to force through a no deal Brexit, will be met by strong and widespread democratic resistance.
"We pledge to work together across parties and across our nations to do whatever is necessary to ensure that the people's voice is able to be heard."
As representatives of the opposition parties met at Church House, Liberal Democrat Brexit spokesman Tom Brake tweeted: ".@BorisJohnson you have done this.
"You've united the parties in the #PeoplesParliament, against your plans to shut the people, and our historic Parliament, out of one of the most critical decisions our country will take; crashing out of the EU without a deal."
"You won't succeed!" he added.