The protest erupted amid a strict coronavirus ban on large gatherings and has become one of the largest street demonstrations since the country’s 2014 military coup.
The student-led protest took place near Bangkok’s Democracy Monument.
Demonstrators cited a list of complaints against the relatively young, year-old government of Thai Prime Minister Prayuth Chan-ocha.
Prayuth was a former army chief who successfully ousted an elected government in 2014.
Protestors had three demands: the dissolution of parliament, an end to harassment of government critics, and amendments to the military-written constitution that critics say virtually guaranteed victory for Prayuth's party in elections last year.
"How can we be OK with the lack of democracy like this?" student activist Tattep Ruangprapaikit told the crowds.
Police were on standby but did not move to stop the protest. The monument was cordoned off with signs reading: "No entry without permission. Maintenance in progress."
Public opposition to Prayuth's rule has been growing in recent months.