Initially, some 500 protesters led by Chumpol Julasai, Natthapol Thipasuwan and Sokonthee Phatthiyakul arrived at the office at noon.
Troops guarding the office set up three layers of barbwires to prevent protesters from storming inside.
Leaders of the protesters demanded that Gen Niphat Thonglek stop allowing caretaker Prime Minister Yingluck Shinawatra to use the office as her temporary office.
At noon, Phutthipong Puttipong Punnakan led several hundreds more of protesters and state enterprise unionists to join the rally.
The Nation