The Wales Millennium Centre aims to deliver arts to the people of Wales.
The key was passed up a long and enthusiastic human chain
And they were certainly involved in the opening ceremony, as a human chain delivered the key to the front door.
The key, which has travelled the world, was taken to Cardiff Bay by boat, before people power helped it along the final few yards.
At the end of the line, the door was opened by mother-of-two Janet Thickpenny, from nearby Barry, on her 40th birthday.
Marilyn Stolz, a human resources manager at Welsh National Opera, was part of the chain and said the building was creating an infectious atmosphere.
Janet Thickpenny and Sir David Rowe-Beddoe with key and padlock
"I think we're all quite excited about it, it's wonderful and the auditorium is fantastic," she said.
"We just all want to be involved in everything. People are volunteering to help out for free tomorrow for the open day."
Further along the line, Alice Davies, from Dinas Powys, said her day had taken an unexpected turn.
"I set out to do a bit of shopping and then remembered it was going to be opening.
She said she thought she would come and have a look, but was surprised to be part of the ceremony.
June Owen, Diana Panniers and Alice Davies at the ceremony
"We were walking along the quay and somebody said there was a human chain and asked us if we wanted to join it," she said.
Her neighbour in the line, June Owen, from Wenvoe, said: "We were doing line dancing this morning - now we're in a line with the key.
"We're very proud to be part of it."
Emily Scott, 10, from Wood Memorial County Primary school in Saltney, Flintshire, had travelled down with classmates after writing a poem for the centre.
"It's called the millennium key and it's about the materials used in the building," she said.
"We wrote it as a class. A poet came in to help us and gave us some words and then we worked on the lines."
Emily Scott travelled from Flintshire for the centre's opening
After seeing the building, she delivered a short, sharp verdict. "It's well good."
After the centre was opened, performances by children's musical groups were held in the foyer.
Harun Miah, an insurance manager who works in Cardiff Bay, was watching the shows, and was proud to be there.
"I have seen it being built because I work right next door," he said.
"It's fantastic, it's a landmark building and the whole area is up and coming.
"The building suits the area."