LAGOS (Reuters) - Nigeria's central bank raised its benchmark interest rate by a surprise 200 basis points to 14 percent on Tuesday and maintained its existing cash reserve ratios for commercial banks in a bid to stabilise the naira. In a Reuters poll, the median forecast of 13 analysts taken July 18-21 predicted that Nigeria would raise interest rates by 100 basis points to 13 percent. (Reporting by Ulf Laessing and Alexis Akwagyiram; Editing by Ed Cropley)