The IRA has formally ordered an end to its armed campaign and says it will pursue exclusively peaceful means.
The IRA statement said it would pursue a peaceful path
In a long-awaited statement, the republican organisation said it would follow a democratic path ending more than 30 years of violence.
Sinn Fein President Gerry Adams said the move was a "courageous and confident initiative" and that the moment must be seized.
Prime Minister Tony Blair said it was a "step of unparalleled magnitude".
"It is what we have striven for and worked for throughout the eight years since the Good Friday Agreement," he said.
The IRA made its decision after an internal debate prompted by Mr Adams' call in April to pursue its goals exclusively through politics.
Mr Adams said Thursday's statement was a "defining point in the search for a lasting peace with justice" and also presented challenges for others.
"It means that unionists who are for the Good Friday Agreement must end their ambivalence," he said.
"And it is a direct challenge to the DUP to decide if they want to put the past behind them, and make peace with the rest of the people of this island."
In a joint communique the British and Irish governments welcomed the statement and said if the IRA's words "are borne out by actions, it will be a momentous and historic development".
"Verified acts of completion will provide a context in which we will expect all parties to work towards the full operation of the political institutions, including the Northern Ireland Assembly and Executive, and the North-South structures, at the earliest practicable date," it said.
KEY POINTS OF STATEMENT
All IRA units ordered to dump arms
Members ordered to pursue objectives through "exclusively peaceful means"
Arms to be put beyond use as quickly as possible
Two church witnesses to verify this
Statement followed "honest and forthright" consultation process
Strong support among IRA members for Sinn Fein's peace strategy
There is now an alternative way to achieve goal of united Ireland
"Volunteers must not engage in any other activities whatsoever"
The Independent Monitoring Commission, which examines paramilitary activity, has also been asked to produce an additional report in January 2006, three months after their next regular report.
During the Northern Ireland Troubles, the IRA murdered about 1,800 civilians and members of the security forces.
The IRA statement issued on Thursday said the end of the armed campaign would take effect from 1600 BST.
"All IRA units have been ordered to dump arms. All Volunteers have been instructed to assist the development of purely political and democratic programmes through exclusively peaceful means. Volunteers must not engage in any other activities whatsoever.
"The IRA leadership has also authorised our representative to engage with the IICD to complete the process to verifiably put its arms beyond use in a way which will further enhance public confidence and to conclude this as quickly as possible."
The statement said independent witnesses from Catholic and Protestant churches had been invited to see the decommissioning process.
It is understood there has already been a meeting between the head of the decommisioning body, General John de Chastelain, and the IRA.
Gerry Adams said the "moment must be seized"
DUP leader Ian Paisley greeted the statement with scepticism, saying that the IRA had "reverted to type" after previous "historic" statements.
"We will judge the IRA's bona fides over the next months and years based on its behaviour and activity," he said.
He said they had also "failed to provide the transparency necessary to truly build confidence that the guns have gone in their entirety".
Ulster Unionist Party Sir Reg Empey, told the BBC's World at One it would take time to convince the people of Northern Ireland that this was more than just rhetoric.
He said: "People are so sceptical, having been burnt so many times before.
SDLP leader Mark Durkan welcomed the statement, saying it was "clear, clean and complete", but "long overdue".
He called on Sinn Fein to commit to the new policing structures in Northern Ireland, as his party had done.
Taoiseach Bertie Ahern has said he welcomes the IRA's statement that it was ending its "armed campaign".
Mr Ahern said the end of the IRA as a paramilitary group "is the outcome the governments have been working towards" since the 1994 ceasefire.
The IRA pledge was welcomed by the United States administration as "an important and potentially historic statement".
A White House statement said the words must now be followed by actions and acknowledged there would be scepticism, particularly among victims and their families.
"They will want to be certain that this terrorism and criminality are indeed things of the past," the statement said.
The statement added that it understood from the IRA communique that "the IRA and its members will no longer have any contact with any foreign paramilitary and terrorist organisations".
When he made his appeal in April, Mr Adams said it was "a genuine attempt to drive the peace process forward".
Republicans had been under intense pressure to end IRA activity after the £26.5m Northern Bank raid in December and the murder of Belfast man Robert McCartney in January.
Political talks last year failed to restore devolution, which stalled amid claims of IRA intelligence gathering at Parliament Buildings, Stormont, in 2002.
The Provisional IRA's campaign of violence was aimed at forcing an end to the British presence in Northern Ireland, leading to a united Ireland.