The incident occurred about 12.20pm on Vibhavadi Rangsit Road, in front of the club which is being used as offices by the Centre for the Administration of Peace and Order (Capo) .
The protesters, led by Luang Pu Buddha Isara, dismantled a barbed wire barricade set up in front of the club grounds.
The monk announced he would give Capo five minutes to send out a representative to talk to him.
After the time lapsed, he walked into the Police Club grounds.
The police warned them through speakers to stop trespassing, saying the protest was illegal.
They then used water canon and fired tear gas canisters at the protesters, who backed away.
The monk was injured and escorted away from the area by guards for first-aid treatment.
Bangkok Metropolitan Administration's Erawan medical centre reported at 1.07pm two men and two women were injured and sent to Vibhavadi Hospital for treatment.