The protesters have three key demands:that Parliament be dissolved,the constitution be re-written and the harassment of government critics ends.
The government of Prime Minister Prayut Chan-o-cha,a former general and junta leader who claimed victory in a March 2019 election that marked a questionable transition back to civilian rule,is in their sights.
But more unusually,Monday's protest called for curbs to the power of the king,who after coming to the throne in 2016 took personal control of several army units and palace assets worthtens of billions of dollars.
Lawyer Arnon Nampa,34,accused the palace of taking on increasing powers that undermined democracy and of inaction in the face of attacks on opponents of Prayut's government.
"Talking about this is not an act to topple the monarchy,but to allow the monarchy to exist in Thai society in the right way and legitimately under a democratic and a constitutional monarchy,"
Thailand has some of the strictest lèse-majesté laws in the world - defaming the monarchy is punishable by up to 15 years in prison - and while none of the protesters at Bangkok's democracy monument were stopped,police warned suspected offences would be investigated.