The IRA said all clandestine units have been ordered to place their weapons in arms dumps and cease all activities, effective 4 p.m. local time, but would not disband, reported the Associated Press.
"All volunteers have been instructed to assist the development of purely political and democratic programs through exclusively peaceful means. Volunteers must not engage in any other activities whatsoever," the IRA command said in a statement to its 500 to 1,000 members, according to the AP.
The IRA arsenal, used to fight for a united Ireland until a 1997 cease-fire, was a major stumbling block to the peace process.
Some 3,600 people died in Northern Ireland since renewed violence began more than 30 years ago, half of them killed by the IRA, according to Reuters.
The group said John de Chastelain, a retired Canadian general who since the signing of a political peace deal in 1997 has been working to disarm the IRA and other militant groups, would be invited to decommission more hidden weapons bunkers soon. It said a Catholic priest and Protestant minister would also be invited to witness the disposal of weapons, reported the AP.
British Prime Minister Tony Blair and his Irish counterpart, Bertie Ahern, hailed the move.
"I welcome the recognition that the only route to political change lies in exclusively peaceful and democratic means," Blair said in his London office. "This is a step of unparalleled magnitude in the recent history of Northern Ireland."
Ahern, who has worked with Blair since 1997 to reach a compromise in the British territory, said the announcement marked "the end of the IRA as a paramilitary organization."
Mistrust of the largely Protestant police force in Catholic neighborhoods and a failure of the political process to move forward were long cited as justification for the IRA's violent activities.
Protestant leaders, who support a continued union with Britain and are suspicious of the IRA's motives, said they would wait several months to see whether the IRA's actions matched its words.
Democratic Unionist Party head Ian Paisley, whose fiery rhetoric and strong pro-British stand has built his party into the largest political group in the region, said IRA commanders "have failed to explicitly declare an end to their multimillion-pound criminal activity, and they have failed to provide the level of transparency that will be necessary to truly build confidence that the guns have gone in their entirety," the AP reported.
All sides publicly say they are committed to rebuilding a joint Catholic-Protestant administration that would replace Britain as the primary government authority in Northern Ireland. But the DUP and other Unionist, or pro-British and largely Protestant groups, have refused to work with Sinn Fein, the IRA-linked party, until the IRA is no longer a threat to stability.
A coalition of Protestant and Catholic moderates gained power in 1999 but fell apart in 2002 amid arguments over IRA activities and arms. Since then, the more strident DUP and Sinn Fein have gained more electoral strength in their communities and the British and Irish governments, seeking to maintain the peace accord, have suspended the Northern Ireland Assembly until the issue of disarmament could be resolved.