Official commemorations of the 40th anniversary of the Aberfan disaster have begun, to remember the 144 people who died in the south Wales town.
A total of 116 children lost their lives when a coal waste tip slid down a mountain, engulfing a school and surrounding houses on 21 October 1966.
First Minister Rhodri Morgan was among those attending a public church service in nearby Merthyr Tydfil.
A private service will be held in Aberfan cemetery on Saturday.
Representatives from Aberfan's memorial committee, parents who lost their children and some survivors were among those attending the public memorial at St Mary's Church, Merthyr Tydfil, on Thursday evening.
During his homily, Father Michael St.Clair said: "Yesterday I was asked several times, what do I think is the need for such a commemoration.
The school and about two dozen houses were covered by the slide
"We pray that all those who lost their lives are brought to that place of peace, light and happiness.
"Why do we commemorate so many things? Because we are a people who remember.
"Why are we here in these valleys? Coal is responsible.
"In those days they didn't realise what they were doing, in our days we are so concerned with global warming and the effect on the environment.
"What can we do to make things better for the future?"
Mr Morgan called on the people of Wales to remember those who died.
He said: "Everyone can remember how they heard about Aberfan.
"I first heard about it on the lunchtime news that day and found it hard to make sense of the horror of it - the fact that a primary school had been buried, that so many children had died and the scale of the rescue operation.
"This was the 1960s. Wales still had 100,000 miners and coal was king. Coal was so important that we all accepted - until Aberfan - that there was a price to pay and we were all prepared to pay it.
Survivor Susan Robertson is pulled from the rubble
"What we never foresaw was that coal could take the lives of our children. That was new, and terrible."
The slip happened just after pupils had arrived for classes at Pantglas Junior School on the last day before half-term that year.
The school and about two dozen houses were covered by the slide, which followed several days of heavy rain.
Images of the tragedy were beamed around the world, prompting donations for a disaster fund which reached a total of £1.75m.
A tribunal found the National Coal Board was responsible for the disaster, but nobody was sacked or prosecuted.
Readings at Thursday's service were given by Councillor Jeff Edwards, who was one of the pupils that survived the disaster at Pantglas.
The Mayor of Merthyr Tydfil, Ian Clark, said: "We pray for the families and friends of all who died that they may draw comfort from their faith and the support of the community."
Cliff Minett, who lost two of his three children in the disaster, said: "It doesn't matter if it's one year or 40 years on - the pain is just the same."