The event was an unprecedented diplomatic effort from Sinn Fein, the former political wing of the IRA and a party that has always refused to take up its seats in Westminster.
The Bill would also stop future inquiries and civil actions related to the Troubles, although it does not fully close the door to criminal prosecutions.
At a rare gathering with a cross-party group of MPs in Westminster on Tuesday night, Ms McDonald called on Tory backbenchers and Peers to speak out against the Prime Minister’s plan to rip up the Protocol.
“We currently have a full onslaught, a full attack against the Good Friday Agreement,” she told the meeting which was mostly attended by left-wing parliamentarians from Labour, the SNP and Plaid Cymru, as well as veteran Conservative Brexiteer Sir Bill Cash.
“And more alarmingly we have an administration, a Prime Minister at No 10 Downing Street who is playing fast and loose, a very dangerous game of brinkmanship, with all that has been achieved over the last quarter of a century.
“That is a most serious and a most grave situation that should call the attention of every member of the House of Commons and every peer in the House of Lords.”
During a speech that drew frequent applause from an audience that included ex-Labor leader Jeremy Corbyn and his former frontbenchers John McDonnell, Richard Burgon and Rebecca Long-Bailey, she insisted Sinn Fein wants good relations with No 10.
She said: “This has to be challenged and not just we’d argue from the opposition benches. This ought to be challenged from within the government itself because it is very, very wrong.
“We don’t want to be poking the eye or picking a fight with Boris Johnson or any of his colleagues but we are left with no option but to challenge this in the most vigorous terms.”
The meeting in Westminster was held after the families of Troubles victims told the Prime Minister the UK Government’s controversial legacy legislation is “an affront to all modern standards of decency”.
Protests also took place in Belfast and Londonderry as MPs debated the legacy plan in the House of Commons, which would offer immunity to people who are deemed to have cooperated with an information retrieval body.
Michelle O’Neill, the vice-president of Sinn Fein, said that most people in Northern Ireland support the Protocol, which created the Irish Sea border and introduced customs checks on British goods.
“The British Government needs to stop pandering to the DUP,” Ms O’Neill, who led Sinn Fein to a historic first majority in the May 5 elections to the Northern Ireland Assembly, told the BBC.
“The DUP’s voice does not reflect the wider view at home,” she said, pointing to the pro-Protocol majority in Stormont after the elections which will make her first minister if power-sharing is restored.
The DUP, which lost its majority, claims the Protocol, which prevents a hard border on the island of Ireland, is pushing up the cost of living and driving a wedge between Northern Ireland and the rest of the UK.
It has refused to enter power-sharing in Stormont, and has blocked the appointment of a new Speaker to the Assembly until the Protocol is removed or replaced in UK-EU negotiations.