SHANGHAI, March 15 (Reuters) - China's central bank rolled over maturing medium-term policy loans while keeping the interest rate unchanged on Tuesday, defying market expectations of imminent monetary policy easing to prop up the cooling economy.

The People's Bank of China (PBOC) said it was keeping the rate on 200 billion yuan ($31.44 billion) worth of one-year medium-term lending facility (MLF) loans to some financial institutions unchanged at 2.85% from the previous operation.

But, as 100 billion yuan worth of MLF loans were maturing on Tuesday, the operation resulted in a net injection of 100 billion yuan in fresh funds into the banking system.

Twenty-nine out of the 49 traders and analysts, or 59% of all participants polled by Reuters, predicted a reduction to the one-year MLF rate.

The central bank also injected 10 billion yuan through seven-day reverse repos to offset same amount of such loans due on the same day, while keeping borrowing costs unchanged at 2.1%, according to an online statement.

($1 = 6.3607 Chinese yuan renminbi) (Reporting by Winni Zhou and Andrew Galbraith; Editing by Kim Coghill)