Security and safety are more and more important in embedded system design. A key issue hence lies in the ability of systems to respond safely when errors occur at runtime, to prevent unacceptable behaviors that can lead to failures or sensitive data leakage. In this paper, we propose a design approach that automatically generates On-Chip Monitors (OCM) during High- Level Synthesis (HLS) of hardware accelerators (HWacc). OCM checks at runtime the input/output timing behavior, the control flow execution and algorithmic properties (via ANSI-C assertions) of the monitored HWacc. OCM is implemented separately from the HWacc and an original technique is introduced for their synchronization. Two synthesis options are proposed to tradeoff between performance and area. Experiment results show that error detection on the control flow is 16x better compared to the existing approaches while the cost of assertions is reduced by 17.48% on average. The impact on execution time (i.e. latency of the HWacc) is decreased by 2.76x at no area penalty and up to 4.5x with less than 10% extra-area. The clock period overhead is at worst less than 5% and the overhead on the synthesis time of the HWacc to generate OCMs is 7.44% on average.