At sea and on land wreaths were cast to remember the 1500 people who died when the Titanic sunk a century ago.
In Belfast, the city that built the Titanic, a memorial garden containing the first-ever monument to list all the victims' names was unveiled during a commemorative service attended by about 300 members of the public.
Earlier, wreaths were thrown into the Atlantic at the site of the wreck from MS Balmoral, a cruise ship that has traced the doomed liner's route across the ocean, while people also held a minute's silence.
And in Halifax, the Canadian port city from where ships sailed to retrieve bodies from the icy Atlantic waters following the sinking on April 15, 1912, 1,000 locals and tourists, some in period costumes, attended a solemn interfaith service at a cemetery where 121 of Titanic's victims are buried. (Twenty-nine more victims are buried in two other local cemeteries).
"Today we gather to remember, not just this legendary ship, but the lives she took with her," said Andrew Murphy, chair of the Titanic 100 Society.
"We also remember our hometown heroes, the volunteers who put their own lives at risk to bring our victims ashore and the people who responded then as we do now with dignity and respect."
The Titanic went down after hitting an iceberg about 800 kilometers (500 miles) southeast of Halifax.
Jane Allen, whose great uncle Thomas Pears went down with the ship as his new wife was rescued, said being aboard MS Balmoral to partake in a Titanic memorial service had been an "incredible" experience.
She told the BBC: "We were all so keen to be at the memorial service. You look down over the side of the ship and you realize that every man and every woman who was not fortunate enough to get into a life boat had to make that decision of when to jump or to stay with the ship until the lights went out."
In total, around 50 people on board the 12-night Titanic Memorial Cruise have a direct family connection to the sinking.
Overnight, the MS Balmoral -- which has traveled from the British port of Southampton-- and the Azamara Journey from New York City approached each other at the site where the Titanic went down to witness a partial reenactment.
The Azamara Journey's captain announced a collision and a commemorative distress call.
"Have struck iceberg. ... We require immediate assistance," read the message. "Have struck iceberg and sinking ... We are putting women off in boats."
The Titanic had been sailing from Southampton on its maiden voyage toward New York when it sank.
In Northern Ireland on Sunday, a few hundred people attended a commemorative service outside Belfast City Hall, among them politicians and relatives of those who died, including a great-great nephew of the ship's doctor.
The Belfast memorial garden contains a nine-metre-wide (29.5-feet-wide) plinth bearing five bronze plaques engraved with the names of the victims.
It is the first time the names of everyone who died has been recorded on one monument. Many existing memorials failed to include the Titanic crew or its musicians.
There is no distinction between first-class passengers and others, with names simply listed in alphabetical order.
US oceanographer Robert Ballard, who discovered the wreck in 1985, was in Belfast and delivered a memorial lecture at the new Titanic Belfast visitor attraction on Saturday.
He spoke about the next 100 years, of preserving the wreck and making it available to all via communications technology, beaming live images from the depths.
The submersible he used to locate the wreck is being refurbished now for another visit in 2013.
Late on Saturday, a commemoration at Belfast's Waterfront hall brought together stars from music and film. There was also a Requiem for the Lost Souls at St Anne's Church of Ireland Cathedral.
In County Mayo, Irish Prime Minister Enda Kenny opened the Mayo Titanic Memorial Park in remembrance of the 11 people from the village of Addergoole who perished in the disaster.
"This memorial park is a testament to the human spirit," said Kenny.
"It tells the story, unique to Western Europe, of the devastating impact the sinking of the Titanic had on a small isolated rural community."
In Halifax, marchers carried battery-powered candles and followed a horse-drawn carriage bearing a casket, stopping at Titanic landmarks.
Warren Ervine, a geological engineer whose uncle Albert at the age of 18 was the youngest member of the Titanic crew, was among the participants.
"My father was always very sad," Ervine recalled. "Like people coming back from the war, they did not want to talk about it. I did not even know he (uncle Albert) was a crew member until 10 years ago. I looked for him on the passenger list."
Bells rang from four churches where the ceremonies for the dead were held in 1912, and the sky lit up as distress flares were fired into the air.
In New York, meanwhile, a range of objects related to the Titanic, from a launch ticket to a piece of rug, were auctioned off Sunday on the 100th anniversary of the oceanliner's sinking, auctioneers Bohnams said.
The sale, entitled "R.M.S. Titanic: 100 years of fact & fiction at Bonhams," included what auctioneers called "an extremely rare original launch ticket with its perforated admission stub still intact." The souvenir of the ship's launch in Belfast May 31, 1911 was sold for $56,250.