About 100 members of the Thai Patriots group on Thursday morning gathered in front of the Ministry of Defence building compound and demonstrated against Prime Minister Yingluck Shinawatra.
The protesters, led by Chaiwat Sinsuwong, blocked the entrance gate of the ministry to prevent Ms Yingluck, who is now also defence minister, and her deputy Gen Yuthasak Sasiprapa from entering the ministry.
Mr Chaiwat said the rally was organised to protect the dignity of soldiers and the armed forces and to protest against the Thaksin regime.
Mr Chaiwat, speaking from a stage on the back of a truck, urged the protesters to block Ms Yingluck from entering the ministry and stop her taking control of the armed forces. He also warned police not to use force against the demonstrators.
Three companies of police were deployed in front of the ministry to maintain law and order.
Police chief Pol Gen Adul Saengsingkaew was reportedly monitoring the situation at the Royal Thai police headquarters.
Ms Yingluck is scheduled to travel to the Ministry of Defence for her first day of work as defence minister at 11am today.