A candlelit choir will perform afloat on the Thames to mark the reopening of the Royal Festival Hall.
The modernisation cost £91m over two years
The iconic Grade I listed building on London's South Bank has undergone a two-year multi-million pound refurbishment programme.
The 2,788-seater auditorium now has upgraded acoustics, under-seat air conditioning and increased leg-room.
About 800 singers will arrive at dusk on Friday and perform a choral work specially composed by Orlando Gough.
The venue will play host to a weekend of free musical events, called The Overture, featuring a cast of 18,000 performers.
There will be performances by the London Philharmonic Orchestra, ballroom and Bollywood dancers, jazz musician Harry Beckett and with a band of 2,500 primary school children and a mass busking session led by Billy Bragg.
"This will be the most ambitious participation weekend we have seen in this country for years," said artistic director Jude Kelly.
"It is an opportunity for artists of all kinds to feel they are the centre of our story."