Friends, family and police are becoming increasingly concerned for Crystal Palace library campaigner Robert Gibson, who has been missing since Monday (June 13).
The 45-year-old was last seen at approximately 3pm on Monday leaving his home in Gipsy Hill to go for a walk.
He may have been in East Croydon around 3.30pm on the same day.
There has been a massive community response in Crystal Palace, with thousands of flyers distributed at train stations by friends and neighbours.
Friends say his disappearance is very out of character and they are becoming more and more worried about his well-being.
He has been a passionate and outspoken campaigner to save Upper Norwood Library since it came under threat of closure.
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Robert is described as white, 6ft tall with a slim build, with grey or silver hair and black-rimmed Prada glasses.
He is possibly wearing a blue or grey T-shirt with green or brown khaki trousers.
Robert is known to frequent the Dulwich and Sydenham areas.
Anyone with information on his whereabouts is asked to call police in Lambeth on 101 or the charity Missing People on 116000.