Protestant leader David Trimble resigned today as Northern Ireland's First Minister, plunging the British province into a political vacuum and threatening a hard-won peace deal with minority Roman Catholics.
In the hours leading up to Trimble's midnight resignation, there were minor clashes between the two sides as the Protestant "marching season," an annual flashpoint for trouble, started in Belfast.
"As of midnight, David Trimble ceased to be First Minister of Northern Ireland's power-sharing government," a Northern Ireland Office spokesman told Reuters.
Trimble, awarded the Nobel Peace Prize winner with Catholic leader John Hume for their part in the 1998 Good Friday peace accord, precipitated the crisis by submitting a post-dated resignation letter several weeks ago in protest at the Irish Republican Army's (IRA) refusal to disarm as part of the deal.
Trimble, who was in France when the resignation came into effect at midnight, appointed Trade Minister Reg Empey, a member of his Ulster Unionist Party (UUP), to take over his duties.
Trimble was attending a commemoration of the First World War Battle of the Somme, in which many soldiers from Northern Ireland died.
Under the landmark Good Friday agreement, the power-sharing government of Catholics and Protestants that Trimble had headed will have a six-week "breathing space," either to re-install Trimble or replace him, before the Northern Irish assembly and executive are suspended.
If such steps fail, Britain can call new provincial elections or re-impose direct rule from London.
As Trimble left the province, police and British troops mounted a strong presence to head off trouble during a parade by the Protestant Orange Order institution.
There were only minor scuffles between police and residents as a concrete and steel barrier was put up by security forces to seal off the Catholic enclave ahead of the march.
Police said the march, part of parades over the next month celebrating victories going back centuries by Protestants over Catholics, went ahead without incident.
Catholics have long denounced the marches as "triumphalism" which stands in the way of peace.
The next flashpoint march is the Drumcree parade in a week's time, when Protestants file past a Catholic enclave in Portadown.
A spokesman for Trimble's UUP said Empey's appointment was intended to "shore up the political institutions and ensure its representation in the government."
Empey said his role would be to perform the functions of First Minister but not take the title or salary.
"This is only a sticking plaster solution for a very short time: for a matter of weeks," Empey said.
Empey said his party would not share power with the IRA's political arm Sinn Fein unless the guerrilla group started to disarm.
Sinn Fein leaders have denounced Trimble's resignation as an evasion of responsibility for peace in the province.
Asked if he wanted Trimble to be re-elected as First Minister Empey said: "That is our hope."
The IRA has said it wants a permanent peace, and security sources say there is no sign of a return-to-war mood in the ranks of the guerrilla group.
It has twice opened up arms dumps for international inspection to prove that the weapons have not been used, but Protestant politicians say that is not enough.