It was another blow to a Good Friday agreement that now seems a long time ago, delivered on the same day 13 years later. Northern Ireland awoke on Friday morning to find that a new group had joined the ranks of armed republican terrorism. Against a backdrop of increasingly violent dissident activity, a statement had been obtained by the Belfast Telegraph heralding the arrival of another player on the scene.

In what is believed to be its first public message, an alliance of former Provisionals promised to embark on a fresh campaign of violence "under the name of the Irish Republican Army". Signalling their capability, they claimed responsibility for the booby-trapped bomb that killed Catholic police recruit Ronan Kerr, 25, in Omagh at the start of the month.

In what represents a major breakaway from Provisional ranks, the group of east Tyrone republicans confirmed what the security and intelligence agencies in Northern Ireland and Britain have known for some time – that the tradition of armed republican resistance is alive, well and active.

Sources point to a recent escalation in both intent and capability among dissident groups. Yesterday the Police Service of Northern Ireland took the unusual step of announcing that dissidents intend to kill more officers "in the coming days and weeks". The PSNI even deemed the situation so critical that it has issued a direct appeal to the public over the Easter holidays to be vigilant, explaining that there is a "severe threat level posed by terrorists".

The latest faction, which is aligned to the Real IRA, includes veteran paramilitaries, some of whom are understood to have been involved in transporting and later detonating the bomb that devastated London's Canary Wharf in 1996. Their manifesto is simple: a continuity of "military operations" until "British occupation" is over.

Ciaran Collins, a 33-year-old from Lurgan, represents a small but growing band of young men in nationalist parts of North Armagh who believe in this cause and that the murder of police and security targets is justified.

The Co Armagh town has been a focal point for dissident violence over the past three years, with repeated attacks on the police, culminating in the 2009 murder of Constable Stephen Carroll, the first PSNI officer to be killed by republican paramilitaries.

Collins says that most of his friends would have supported the policeman's murder. "When they heard about it, they would have had no problem with it. I had no problem with it. It might have been sad for him and his family, but the attack was against what his uniform stands for – British rule in the north of Ireland."

Collins claims such attacks increase recruitment to the dissident republican forces. "There are people joining from all classes and backgrounds who are fed up with the lies of the so-called peace process, including third-level students," he says.

Intelligence assessments cite the province's parlous economy as a further motive for a cohort of disenfranchised youth now attracted to dissident groups, although their leadership remains comprised of former Provisionals.

Dolores Kelly, a local SDLP councillor, was attacked by young dissident republicans last week as she was driving through the nationalist Kilwilkie estate in Lurgan. She sustained cuts to her hands after rioters attempted to smash her car windscreen.

"There are about 30 to 50 young people whom the Continuity IRA have recruited in this area, and they are the hardcore who are causing the trouble," she says. "Many of these incidents are caused by young men who have been drinking and taking drugs. Their violence is recreational and is being exploited by more sinister forces."

Sinn Féin, meanwhile, insists with some justification that it continues to represent the huge majority of northern nationalists. The dissidents, they insist, are an unrepresentative rump of fundamentalists without a coherent political strategy, offering little to nationalist communities bar "civil administration" – otherwise known as punishment beatings.

The party is banking on repeating its recent successes in the Irish general election in next month's assembly election. Sinn Féin supporters are hoping the party will edge further ahead of its SDLP rivals and could even become the largest party in Northern Ireland on 5 May – the 30th anniversary of the death of Bobby Sands, an anniversary viewed with some trepidation by security sources.

The nightmare scenario for the security forces is that the violence which has been concentrated so far in Derry, Belfast, Omagh and Lurgan will spill over into Britain. Internal government security data underlines the disturbing spike in violent activity, with 15 reported terrorist incidents in 2007, rising to 40 for last year; this year's running total is nine.

Most attention is falling upon the Real IRA and the smaller but technically able Oglaigh na hEireann, who have improved their explosives technology over the past two years. Yet security forces on both sides of the Irish border and of the Irish Sea take some comfort in the fact that the anti-ceasefire republican groups remain divided and geographically scattered. The Real IRA's main base is Derry, while Continuity IRA (which is aligned with Republican Sinn Féin) has a presence in Lurgan. Meanwhile, ONH has units in Co Tyrone and north Belfast. Thus far there is no united, coherent command structure, though most members are familiar with one another. The current collective membership of republican dissident groups is placed in the "mid-hundreds", although it is the increasing capability, not numbers, that matters.

Moreover, relations between groups can be fractious, even within one organisation. Last week a Real IRA representative said it had fallen out with Continuity IRA over the latter group's claim to be the true, sole inheritor of the Irish republican tradition. There can also be confused lines of communication. Last week the Irish state broadcaster, RTE, ran an interview with a masked man claiming to be from the Continuity IRA warning about an escalation of war. Yet in a coded communique to the Observer on Good Friday, the CIRA terror group leadership dismissed the spokesman as an "imposter".

For all the occasional amateurishness and in-fighting, though, extreme republicanism has not been this buoyant and self-confident for a decade. And not all the dissidents' recruits are young disaffected men such as those in Lurgan. Josephine Hayden, with her dyed black bobbed hair and sharp-rimmed glasses, resembles a retired lady who lunches rather than a firebrand republican committed to destroying the political settlement in Northern Ireland.

Yet when it comes to the Queen's visit to Dublin next month, the pensioner sounds as uncompromising and militant as the young men who frequently hurl homemade bombs at police in some of the most disadvantaged areas north of the Irish border. Folding away T-shirts commemorating the 30th anniversary of the H-block hunger strike, the 65-year-old is blunt about what should happen to her majesty on her historic visit to the Irish Republic next month.

Asked about her attitude to a sniper firing on the royal entourage, Hayden says: "I wouldn't have any problem with it. I think it would be justified, most definitely, because she is the chief of staff of the British armed forces, who are still occupying our country, who are still operating on Irish streets in the six counties of Ulster. "You might say that she is just a little old grandmother, but it is what she represents, what she symbolises, that counts. She is a legitimate target, yes."

Pressed on the use of such a phrase – a term the Provisional IRA frequently deployed during the Troubles – to describe the Queen, Hayden is unapologetic. On PC Kerr's murder and the national outrage it provoked, Hayden applies the same justification to the 26-year-old's murder: "He too was a legitimate target because he put on a British uniform."

Hayden, who served four-and-a-half years in Limerick jail for possession of arms for the Continuity IRA, accepts that most people in Dublin and across the Republic would be horrified at the idea of a gun or bomb attack on the Queen. But she stresses that recalcitrant republicans have "always been a minority". Inside Republican Sinn Féin's drab headquarters in Dublin's Parnell Street, she seems to revel in being part of an unforgiving band of hardliners.

Tomorrow Hayden and her RSF comrades will pay homage to the republican fallen, from the 1798 rebellion to the Irish War of Independence, at the nearby Garden of Remembrance. In a fortnight's time the Queen will come to the same spot as part of her three-day tour of Ireland. The Observer has also learned that another radical republican group opposed to Sinn Féin's peace strategy, known as Éirígí (Irish for Rising), plans to occupy the garden on 15 May, two days before the Queen and the Duke of Edinburgh arrive. Éirígí intends to erect a "republican protest camp" inside the garden, with the expectation that the Garda Síochána will be forced to remove them, an incident that could trigger disorder during the royal tour.

The signs are disturbing. The armed republican tradition appears intact, despite being more isolated and cut off from the Irish political mainstream than ever. The leftist urban terrorism of the Baader-Meinhof group in Germany or Action Directe in France proved to be a brief violent backwash from 1960s radical protest and is fading into memory. Even the IRA's old allies in Eta have abandoned their armed struggle in Spain.

Yet, with just five years to go before the 100th anniversary of the 1916 Easter Rising, violent republicanism is renewing itself in a way that may soon be causing as much concern in Britain as it already is in Northern Ireland.