BELFAST, Northern Ireland — Queen Elizabeth II and a former Irish Republican Army commander offered each other the hand of peace Wednesday in a long-awaited encounter symbolizing Northern Ireland’s progress in achieving reconciliation after decades of violence.

The monarch and Martin McGuinness met privately inside Belfast’s riverside Lyric Theatre during a cross-community arts event featuring Northern Ireland musicians, poets and artists.

Media were barred from seeing their first handshake during an ice-breaker over coffee and tea. But the two shook hands again a half-hour later for the cameras’ benefit, documenting a moment that would have been inconceivable back in the days when IRA leaders were plotting to kill the British royal family. McGuinness’ Irish nationalist Sinn Fein party had never attended a royal function before.

Underlying the sensitivity of the occasion, no live footage or sound was permitted to be broadcast. Outside, flak-jacketed police shut down all roads surrounding the theater and told residents to stay inside their homes.

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Both smiled broadly as McGuinness took the queen’s white-gloved hand and spoke to her for about 5 seconds. Afterward McGuinness said he had told her in Gaelic — a language neither of them speaks — ‘‘Slán agus beannacht’’ and told her this meant ‘‘goodbye and godspeed.’’ The latter word actually means ‘‘blessing.’’

The 86-year-old head of state, resplendent in a suit and broad-brimmed hat of matching apple green, didn’t speak but kept smiling as she shared a stage with a man linked to the killing of her cousin Lord Louis Mountbatten. Experts on Irish republicanism say McGuinness, 62, was the IRA’s chief of staff when the outlawed group blew up Mountbatten’s yacht in 1979, killing the 79-year-old and three others.

McGuinness quickly left afterward. ‘‘It went really well. I’m still a republican,’’ he said in response to a reporter’s question as he stepped into his chauffeur-driven government car.

The queen — in Belfast officially to celebrate her 60th year on the throne with an open-air party attended by more than 20,000 royalists overwhelmingly from the Protestant majority — also received a gift from Northern Ireland’s unity government that McGuinness leads alongside a Protestant, Peter Robinson. Their unlikely but surprisingly stable coalition is the central achievement flowing from Northern Ireland’s 1998 peace agreement and the IRA’s 2005 decision to renounce violence and disarm.

But McGuinness stepped back as Robinson presented the gift, a delicate woven porcelain basket made by renowned Northern Ireland pottery firm Belleek. Sinn Fein officials said they didn’t want McGuinness pictured offering a present to the queen.

The event marked the latest, perhaps ultimate, moment in Northern Ireland peacemaking that has delivered a series of once-unthinkable moments of compromise.

Irish republicans long had rebuffed invitations to attend British royal events that occur regularly in Northern Ireland. But analysts said McGuinness’ U-turn became inevitable once the queen made her first state visit to the Republic of Ireland in May 2011, where she won public acclaim and made generous gestures, including honoring Irish rebels who died fighting for independence from Britain. Sinn Fein’s leaders suffered sharp criticism for boycotting her visit.

Diplomatic pressure built on Sinn Fein after McGuinness, who ran unsuccessfully last year to be the Republic of Ireland’s president, said on the campaign trail that he would meet the queen if elected. He came third.

The winner of that election, President Michael D. Higgins, accompanied McGuinness in meeting the queen. ‘‘The exchange of greetings and courtesies that took place this morning marks another important step on the journey to reconciliation on this island,’’ Higgins said.

Peter Sheridan, director of Cooperation Ireland, the cross-border charity that organized Wednesday’s meeting, described the atmosphere as unexpectedly low-key and relaxed given that it was “a seminal event’’ in relations between both parts of Ireland and Britain.

Sheridan, a former senior Northern Ireland policeman, accompanied the queen and McGuinness as they inspected a row of painted portraits of figures from the Northern Ireland arts scene and chatted with two of the subjects, poet Michael Longley and pianist Barry Douglas. Sheridan said he was struck by ‘‘the very ordinariness of it, even if it was not ordinary people.’’

While the queen showed no sign of unease, the same couldn’t be said of her 91-year-old husband, Prince Philip, who is well-known for blunt speaking and the occasional diplomatic gaffe. At one point, as McGuinness sought to engage him in conversation, Philip quickly shot ahead to his wife’s side. And at the farewell, McGuinness got only a quick, silent handshake from Philip, who was Mountbatten’s nephew.

Later, Elizabeth visited Northern Ireland’s big new tourist attraction, a four-story interpretive center about the doomed sea liner Titanic that was constructed in 1911 in Belfast’s shipyards. Welcoming her, the Protestant power-sharing leader Robinson demonstrated how much Northern Ireland’s majority community values the British monarchy. McGuinness didn’t attend.

‘‘I know for many in the media the focus has been on a handshake and a photograph, but for most people in Northern Ireland it is not about one moment of history but the opportunity to celebrate and give thanks for 60 very full years of your majesty’s service to this nation,’’ Robinson said.

‘‘If I may give voice to the oft-declared desire of the people of Northern Ireland, on whose loyalty and support you can always depend: Long may you reign over us,’’ he said.

The monarch and Martin McGuinness met privately inside Belfast’s riverside Lyric Theatre during a cross-community arts event featuring many of Northern Ireland’s top musicians, poets and artists. Media were barred from seeing their first handshake, but the two shook hands again a half-hour later for a TV camera and two photographers.

Underlying the sensitivity of the occasion, no live footage or sound was permitted to be broadcast. Outside, flak-jacketed police shut down all roads surrounding the theater and told residents to stay inside their homes.

The first soundless TV footage showed a serious-faced McGuinness walking, hands behind his back, behind the queen as she met poet Michael Longley and pianist Barry Douglas in front of newly painted portraits of them and other Belfast artists. Also in the group was McGuinness’ Protestant colleague atop Northern Ireland’s unity government, Peter Robinson; the head of state of the Republic of Ireland, President Michael D. Higgins; and the queen’s husband, Prince Philip.

Then, more delayed footage showed McGuinness and Robinson standing first in line to shake the queen’s hand, then Philip’s. McGuinness and Elizabeth exchanged smiles and brief pleasantries.

McGuinness said he told the queen, in Gaelic, ‘‘Goodbye and godspeed,’’ and translated the phrase for her. She didn’t appear to say anything, just smiled and listened.

The event marked the latest, perhaps ultimate, moment in two decades of Northern Ireland peacemaking that have delivered a series of once-unthinkable moments of compromise.

Experts say McGuinness, 62, was the IRA’s chief of staff when the outlawed group assassinated the queen’s cousin, Lord Louis Mountbatten, in 1979, one of the most high-profile victims of a four-decade conflict that has claimed 3,700 lives.

The IRA formally abandoned its campaign to force Northern Ireland out of the United Kingdom and disarmed in 2005. Two years later, McGuinness became the senior Catholic politician in a new unity government, the central objective of Northern Ireland’s 1998 peace accord. His coalition with Robinson has governed Northern Ireland in cooperation with Britain in surprising harmony since.

McGuinness’ Irish nationalist party, Sinn Fein, had refused all contact with British royals until Wednesday. Its leaders were heavily criticized last year for boycotting the queen’s first-ever state visit to the Republic of Ireland, a widely celebrated event that demonstrated overwhelming Irish desire for strong relations with Britain.

The queen came to Belfast on Wednesday as part of UK-wide celebrations of her 60th year on the throne. She is scheduled later in the day to see the city’s new Titanic exhibition and attend an open-air party involving more than 20,000 locals at Stormont, the hilltop base for Northern Ireland’s power-sharing government.

IRA die-hards opposed to the group’s 2005 decision to renounce violence and disarm sought to express their disapproval of the queen’s visit before she arrived.

Police said nine officers were injured, none seriously, during overnight rioting on the edge of Catholic west Belfast. They said a crowd of about 100 teens and young men bombarded police units with 21 petrol bombs and other makeshift weapons. No arrests were reported, though police cameras videotaped the masked, hooded attackers in hopes of identifying them later.

And in a separate confrontation Tuesday night, one Catholic man was hospitalized after rival Protestant and Catholic groups clashed on the hilltop overlooking Catholic west Belfast. The Protestants were trying to vandalize a massive political display erected by the Catholics featuring an Irish flag and a slogan rejecting the queen.