The Irish Republican Army, which waged a three-decade-long paramilitary struggle against British rule in Northern Ireland, formally ordered an end to its armed campaign Thursday and pledged to pursue its political aims through "exclusively peaceful means."

The outlawed organization, in a long-awaited statement, said it would participate in the democratic process and no longer engage in violent activities.

The IRA was blamed for around 1,800 of the more than 3,000 deaths during the period known as "The Troubles," an era of conflict beginning around 1970 and lasting until the mid-1990s.

"All IRA units have been ordered to dump arms," the organization said in a statement. "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 said the order would take effect at 4 p.m. (11 a.m. EDT). It said that "two independent witnesses" from the Protestant and Catholic churches have been invited to verify the scrapping of IRA weapons. Over the years, the IRA has amassed tons of arms and explosives, and much of the arsenal is believed to hidden in bunkers in Ireland.

The announcement was welcomed by British Prime Minister Tony Blair and moderate political leaders, but it was greeted with skepticism by IRA opponents, including staunchly unionist Protestant leaders in Northern Ireland.

In Washington, the White House issued a statement hailing the IRA announcement as "important and potentially historic." Presidential spokesman Scott McClellan said that consistent with the IRA's pledges, "we understand that the IRA and its members will no longer have any contact with any foreign paramilitary and terrorist organizations."

McClellan added: "This IRA statement must now be followed by actions demonstrating the republican movement's unequivocal commitment to the rule of law and to the renunciation of all paramilitary and criminal activities. We understand that many, especially victims and their families, will be skeptical. They will want to be certain that this terrorism and criminality are indeed things of the past."

Sen. Edward M. Kennedy (D-Mass.) said, "Hopefully, this statement means we're finally nearing the end of this very long process to take guns and criminality out of politics in Northern Ireland once and for all." He said in a statement that he looks forward to verification that "paramilitary activity and criminality have ended. . . ."

Blair said the IRA had taken a step of "unparalleled magnitude."

"I welcome the statement of the IRA that ends its campaign, I welcome its clarity, I welcome the recognition that the only route to political change lies in exclusively peaceful and democratic means," Blair told reporters in his Downing Street office, Reuters news agency reported. Blair said the step paved the way for the revival of a power-sharing government in Northern Ireland and raised hope that it could "banish the ghastly, futile violence of Northern Ireland forever." He called on the IRA to scrap its weapons as soon as possible.

Ulster Unionist leader Reg Empey said, however, that he could not take the IRA statement at face value because of "so many false starts in the past," Reuters reported. "Naturally people are going to say actions speak louder that words," he said. "Let's see what happens to the weapons. Let's see what happens on the ground."

Empey and other unionist leaders noted that the IRA announcement did not say the organization would disband, did not explicitly refer to ending criminal activity and stopped short of accepting the legitimacy of Northern Ireland's police force. Nor did the IRA's statement indicate when its weapons would be decommissioned.

"We've heard it all before," firebrand unionist leader Ian Paisley said in a BBC Television interview. "You can wrap it up any way you like . . . but we want the action, the proof this is happening."

The IRA has been fighting to unite Northern Ireland, which has a Protestant majority, with predominantly Catholic Ireland. The British government and Northern Ireland's Protestants have battled to keep the territory part of Great Britain.

A 1998 accord, known as the Good Friday Agreement, largely ended three decades of sectarian violence in Northern Ireland and set a goal of power sharing in the territory.

Sinn Fein, a political party allied with the IRA and a signatory to the Good Friday Agreement, called on the IRA in April to end its armed struggle following a series of highly publicized crimes.

In December, the IRA was accused of robbing a Belfast bank of a record $50 million. A month later, IRA members stabbed to death a Catholic civilian, Robert McCartney, outside a Belfast bar, a murder that triggered widespread outrage and prompted the victim's family to wage an international campaign for justice.

While the IRA statement did not explicitly address criminality, and the organization has said its activities should never be described as crimes, some observers noted that its order for IRA members not to "engage in any other activities whatsoever" could be taken to mean criminal rackets that have included money laundering, counterfeiting and smuggling, in addition to bank robbery and violent retribution against opponents in Catholic areas.

The statement made clear, however, that the IRA was not renouncing its ultimate goal or the legitimacy of its past armed struggle.

Calling for "full implementation of the Good Friday Agreement," the statement said that "our decisions have been taken to advance our republican and democratic objectives, including our goal of a united Ireland. We believe there is now an alternative way to achieve this and to end British rule in our country."

It said the IRA remains "very mindful of the sacrifices of our patriotic dead" and of members who went to jail. "We reiterate our view that the armed struggle was entirely legitimate."

Branigin reported from Washington.