Hong Kong police said on Monday they are investigating reports that a man was set on fire during the violent protest in the city.

Videos appeared online showing a man dressed in black pouring liquid over a second man and setting him on fire following a heated argument on a footbridge.

The man has been hospitalized and is in critical condition, according to local reports. 

It's a very malicious case of setting an individual on fire in front of many witnesses, said Hong Kong Special Administrative Region Chief Executive Carrie Lam at a press conference on Monday. 

"This is a blatant breach of peace and rule of law and totally inhumane act that nobody should condone," she said.