After a night of violence throughout Northern Ireland that left at least 100 persons injured, half of them police officers, the most important day in the Protestant "marching" calendar passed in relative peace today.

As darkness fell tonight, however, new street clashes began between Protestant and Catholic youths, and between police and Protestants in a number of areas.

In a separate development, the Royal Ulster Constabulary, the province's police force, announced tonight the suspension from duty of two men, believed to be senior officers. The suspensions, according to a terse constabulary announcement, were based on recommendations of an independent British police inquiry into an alleged RUC "shoot-to-kill" policy against suspected Catholic terrorists that left six unarmed men dead in 1982.

Tens of thousands of Protestants took to the streets in 19 towns and cities in the province this morning to commemorate their forefathers' victory over the Catholics in the 1690 Battle of the Boyne. Organized by the Orange Order, the Protestant civic and social lodges that blanket the province, the marches are lengthy processions of fife and drum bands and lodge members wearing traditional bowler hats and carrying furled umbrellas.

Painted banners carried at the head of each lodge group commemorated battles won by William of Orange against the Catholic James II nearly 300 years ago. Many had slogans such as "Defense Against Popery;" most included a depiction of William himself, dressed in a red coat and plumed hat, astride a prancing white charger with his sword held aloft.

March routes were fortified with a strong police presence, backed by British Army troops. But the atmosphere in most places was festive, more akin to American Independence Day parades than the battlegrounds British authorities had feared.

Catholic political leaders, however, sharply protested a police decision late last night to allow Protestants to march today through Catholic neighborhoods in Portadown, a partial reversal of a police ban announced last week.

The decision seemed likely to set back government efforts to convince minority Catholics that the predominantly Protestant constabulary is a nonsectarian force willing to defend their interests.

"Depoliticizing" the constabulary, and overcoming widespread Catholic suspicion based on incidents like those under the alleged "shoot-to-kill" policy in 1982, are among the principal goals of the Anglo-Irish agreement signed last November by the governments of Britain and Ireland. The agreement, which gives Dublin a consultative voice in running the province on behalf of the Catholics, is despised by the Protestant majority, which has vowed to destroy it.

The rerouting of last year's Portadown march led to the first of what have now become regular clashes between Protestants and police that have increased in ferocity and frequency -- as have killings of police and soldiers by the Irish Republican Army since the agreement was signed.

John Hume, leader of the predominantly Catholic Social Democratic and Labor Party, called the police reversal on this year's Portadown parade a "victory for the bully-boys and cudgel carriers."

Brian Lennon, a Catholic priest in Portadown, said that "people are furious" over the decision. "There is no respect being shown for the Catholic or nationalist identity in this province."

Catholics living along the Garvachy Road in Portadown stayed indoors, their sidewalk lined shoulder to shoulder with police, as about 400 Orange Order marchers walked by this morning.

Although the parade passed without incident, surrounding streets still were littered with the debris of a night of rioting.

After the Portadown march local Orangemen and their bands headed for another parade in nearby Armagh, the county seat. There, the atmosphere was noticeably cheerful, with thousands of families turned out in their best clothes to cheer.

Even at the most festive parades, however, there was little willingness by many Protestants to forget what they see as the threat posed by the Anglo-Irish agreement.

Most seemed to share the views of Jack Hobson, a retired milkman, and his wife Doris who positioned their lawn chairs on a traffic island in the middle of Armagh's main street today for a good view of the march.

"There is no use in beating around the bush," Doris Hobson said. The purpose of the agreement -- although heatedly denied in both Dublin and London -- "is to get a united Ireland" under Catholic rule from Rome. The British government, she said, "is trying to sell us out."

Jack Hobson agreed. The Protestants, he said, would stand up for their rights, even if it took violence: "We've done it before."