The leader and other members of the anti-monarchist group Republic have been released from detention after being held for hours during King Charles III's coronation on Saturday, raising doubts whether the police response was proportionate, Reuters reports.

Police arrested the leader of the group, Graham Smith, and 51 others in central London as thousands of royal fans came to see the coronation of King Charles on Saturday, saying their duty to prevent riots outweighed their right to protest.

Republic said the detained members began to be released late Saturday night after nearly 16 hours in detention.

Police said on Saturday they understood the public's concern following the arrests, but stressed they acted after receiving information that the protesters showed up determined to disrupting the coronation.