BELFAST, Northern Ireland — The leaders of Northern Ireland’s major Protestant and Catholic parties, sitting side by side for the first time in history, announced a stunning deal yesterday to forge a coalition of archenemies within six weeks.

“We all saw something today that people never, ever thought would happen,” said British Secretary of State Peter Hain, who expects to hand power May 8 to a coalition led by the polar opposites of provincial politics: Ian Paisley’s Democratic Unionists and Gerry Adams’s Sinn Fein.

Mr. Paisley, a Protestant evangelist who for decades has sought to thwart compromise with Roman Catholics, sat at a table beside Adams, a reputed Irish Republican Army veteran whom Mr. Paisley long denounced as a “man of blood.” Throughout the tortuous 14-year course of Northern Ireland’s peace talks, Mr. Paisley had never before agreed to negotiate directly with Mr. Adams.

Their agreement, after barely an hour of discussions in the lawmakers’ dining hall in Stormont Parliamentary Building in Belfast, called for Sinn Fein and the Democratic Unionists to work directly together on a detailed program for government.

Britain, in turn, promised to pass emergency legislation today that would extend its deadline for a working power-sharing government from yesterday to May 8. On that date, the Northern Ireland Assembly would elect a 12-member administration with Mr. Paisley at its head and Sinn Fein deputy leader Martin McGuinness in the no. 2 post.

Messrs. Paisley and Adams largely looked at their scripts, not each other, as they addressed live TV audiences across Britain and Republic of Ireland. Both agreed they must leave behind Northern Ireland’s bitter divisions and forge a unity government, the central goal of the Good Friday peace pact of 1998.

“We must not allow our justified loathing of the horrors and tragedies of the past to become a barrier to creating a better and more stable future for our children,” said Mr. Paisley, 80, whose party previously boycotted contact with Sinn Fein because of its links to the outlawed Irish Republican Army.

“In looking to that future, we must never forget those who have suffered during the dark period from which we are, please God, now emerging,” Mr. Paisley said. “We owe it to them to craft the best possible future.”

Mr. Adams, 58, a reputed veteran IRA commander who wore a white Easter lily pin in honor of the dead from a 1916 rebellion against British rule, said the accord “marks the beginning of a new era of politics on this island.”

“The relationships between the people of this island have been marred by centuries of discord, conflict, hurt, and tragedy. In particular this has been the sad history of orange and green,” Mr. Adams said, using the local labels for the British Protestant and Irish Catholic sides of the community in Northern Ireland.

“Now, there’s a new start, with the help of God,” he said.

The conflict over Northern Ireland, a corner of Britain with 1.7 million residents, has claimed more than 3,600 lives since the 1960s — when Mr. Adams was an up-and-coming IRA member from Catholic west Belfast; Mr. Paisley was the province’s most infamous opponent of a Catholic civil rights movement.

By the mid-1980s, Paisley campaign posters pictured him carrying a sledgehammer and vowing to “smash Sinn Fein.” That party had begun contesting elections as part of Mr. Adams’s strategy to end his diplomatic isolation and turn IRA die-hards toward politics.

Britain and Ireland have spent 3 1 /2 years trying to coax the Democratic Unionists toward Sinn Fein following their triumph in 2003 assembly elections.

Prime Ministers Blair of Britain and Ahern of Ireland appeared to have forced yesterday’s breakthrough by declaring March 26 their “unbreakable” deadline: Either the Democratic Unionists would agree to cooperate by that date, the threat went, or Britain would abolish the assembly — something neither side wanted.

While yesterday’s deadline was missed, others agreed that the Democratic Unionists’ dramatic decision to drop their boycott on peace talks with Sinn Fein was worth it.