A petition containing 3,721 names was handed to Croydon Council on Monday night in a bid to persuade it to increase funding for Upper Norwood Library.
Upper Norwood Councillor Pat Ryan handed over the document and called on cabinet member for sport and leisure, Councillor Steve Hollands, to give the library more cash.
There were cheers from campaigners sitting in the public gallery as Coun Ryan pleaded their case.
He said: "The library has been strangled by financial constraint. It's absolutely disgraceful. If the situation doesn't change, two full time members of staff will be made redundant and there will be a 30 per cent reduction in operating hours."
The library in Westow Hill receives funding from Lambeth and Croydon Councils, but protestors claim money from both authorities is not sufficient.
If funding is not increased, the library, which was refurbished at a cost of £1.1million a year ago, could face closure.
Many people have joined the campaign to save the library including Tessa Jowell, Secretary of State for Culture, Media and Sport, and Croydon North MP Malcolm Wicks.
Leader of Croydon Council, Councillor Mike Fisher, accused Lambeth Council of "playing games".
He said new financial arrangements needed to be put in place at a level both authorities could afford.
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