LONDON (Reuters) - King Charles thanked singers who performed at a coronation concert in his honour at Windsor Castle for a "great treat" as a long holiday weekend of celebrations drew to a close on Monday.

Amid pomp and pageantry, Charles and his wife Camilla were crowned at London's Westminster Abbey on Saturday in Britain's biggest ceremonial event in 70 years.

Celebrations on Sunday culminated in the "Coronation Concert" featuring singers including Lionel Richie and Katy Perry, as the royals joined a 20,000-person audience at Windsor, the king's palace to the west of London.

In a surprise comedy sketch after the concert, Charles and Camilla interrupted Richie and Perry as they were relaying their experience of the weekend on TV show American Idol, on which the singers are judges.

Charles asked Richie whether the singers will be there "all night long", in reference to one of Richie's hits.

"I just wanted to check how long you'll be using this room for?" asked a chuckling Charles.

"Thank you so much for your brilliant performance, and Katy, it was wonderful, it really was. A great treat to have you both here."

After weekend of street parties and celebrations, Monday, which is a public holiday in Britain for the coronation, sees thousands of organisations get together for a volunteering programme.

Members of the royal family, Prime Minister Rishi Sunak and leader of the opposition are expected to volunteer at "The Big Help Out" events around the country.

(Reporting by Alistair Smout; Editing by Alison Williams)