The new Swiss Light, financed by the Swiss foreign ministry, was turned on at a late night ceremony on Thursday ending a day of celebrations for the opening of the world's largest modern art museum, Tate Modern.
The light on top of the museum's 110-metre high chimney is apparently visible from all over London, and sends a shaft of light down through a special glass roof into the museum.
It was financed by the Swiss government-sponsored Commission for the Presence of Switzerland Abroad at a cost of SFr750,000, and according to the Swiss foreign ministry "it is intended to be constant reminder of the Swiss presence in London".
The light was turned on after a jamboree party with 4,000 diplomats, officials, artists and celebrity guests, including musicians Mick Jagger and Madonna.
Tate Modern, designed by the Swiss architects, Herzog and de Meuron, was opened by Queen Elizabeth on Thursday. The architects have received almost universal praise for their work on converting the former power station into a massive, light-filled SFr350 million museum, facing St Paul's cathedral on the other side of the Thames.
The Swiss interior minister, Ruth Dreifuss, who was the Swiss government's representative at the opening ceremony, described Tate Modern as a "wonderful and very important museum". She said she was "proud that so many Swiss, and so many Swiss traditions, have had an influence on Tate Modern".
However, Dreifuss stressed that with the exception of the Swiss Light at the top, there was no official involvement. She described the Swiss contribution as "the cherry on the cake" but emphasized the magnificence of the building itself.
She also paid tribute to the landscape work, still going on around Tate Modern, which was designed by another Swiss firm, Kienast Vogt.
One of the architects, Jacques Herzog, said many doors had opened as a result of the Basel-based company, Herzog & de Meuron, being commissioned to design the new museum. He said they should be viewed as international architects, who were in Switzerland but were international at the same time.
"Being based in Basel it has always been very natural to be going beyond borders, working in different languages", he said.
Lars Nittve, the Norwegian-born director of Tate Modern, told swissinfo Herzog and de Meuron had been his favourite architects for some time, and that the success of their latest project proves he was right to support them.
Both Herzog and his partner, de Meuron, spoke of how they had scrapped the interior of the former power station without changing the exterior. From the outside the building still resembles a power station. None of its 4.2 million bricks have been touched and the chimney has been retained.
The main addition to the exterior is a partial glass roof that has a dramatic impact on lighting within the museum.
The museum is open to the public as of Friday. An estimated two million people a year are expected to visit Tate Modern, but a Swiss official said that judging by initial interest that figure might well be surpassed.
by Ron Popper