Sinn Féin leader Gerry Adams said the power arrangement between the Conservative government and the DUP will be a "coalition for chaos" that threatens peace in Northern Ireland.
He spoke as Theresa May's government continues to negotiate terms for a supply-and-confidence deal with Mr Adams' party's chief political opponents to help the Tories command a majority in the House of Commons.
"I would hardly call that sort of arrangement ... stable," Mr Adams told reporters at a news conference as he used the much-repeated Tory campaign slogans against them.
"We don't believe that any deal with the DUP here and English Tories will be good for the people here," he added as he warned landmark peace talk agreements were under threat.
"Any deal which undercuts in any way the process here or the Good Friday and the other agreements is one which has to be opposed by progressives," he said as he stood in front of the party's seven new MPs.
The Sinn Féin leader, who has confirmed his party's elected members will not take their seats in Parliament, called on both the outgoing and incoming Taisoeach to be prepared to act on their expressed concerns over the DUP's pivotal new role in Westminster.
He noted it was an "interesting time" in both British politics and Irish politics with the Brexit "train coming down the track".
Mr Adams praised Sinn Féin's "magnificent seven" MPs for a "very good election" and also congratulated the DUP on the 10 MPs they secured after the collapse of talks over the Northern Irish power-sharing executive.
Addressing the ongoing domestic gridlock, he said his party would be prepared to return to power at Stormont today or tomorrow if all rights issues were met.
But he added that DUP leader Arlene Foster should resign her position and not feel emboldened by the Conservative talks.
Mr Adams said: "We need a united executive that has a long-term strategic view that is to the interest of all (Northern Irish citizens)."