Student groups have rallied nearly daily around the country since July 18, calling for an end to military influence over Thai politics after a disputed election last year kept junta leader Prayuth Chan-ocha as prime minister five years after he first took power in a 2014 army coup.
Demonstrators want a revised structure and are also contacting for reform of the monarchy - a delicate subject in Thailand.
There have been nearly daily student-led demonstrations in recent weeks.
Progressive Movement leader Thanathorn Juangroongreangkit said earlier on Friday that the anti-government protesters' demands should be on the table for talks, as crackdowns will not solve the country's political problems.
During yesterday's demonstration, which drew a diverse crowd of all ages, many said they agreed with the students' demands.
"We want a new constitution that is really democratic and we want that to lead to a democratic election and we want a democratically elected prime minister", she said. "Lastly, our dream is to have a monarchy which is truly under the constitution".
By Sunday evening the protesters - who are demanding major democratic reforms - had taken over the busy intersection around Bangkok's Democracy Monument, which was built to mark the 1932 revolution that ended royal absolutism.
"Down with feudalism, long live the people", protesters chanted.
Prayuth won elections a year ago that the opposition says were held under rules to ensure that he kept power.
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The BBC's Jonathan Head in Bangkok states current inclusion of the monarchy in the protesters' demands has electrified the discussion.
Prime Minister Prayuth Chan-ocha, a former general, says the majority of Thais do not support the protesters.
The government may also be faced with a dilemma, since it is committed to defending the royal institution but likely wary of acting with too heavy a hand that might tilt public support to the protesters.
But the military had taken steps to entrench its political role, and the election saw Mr Prayuth re-installed.
Anti-government protests have spread across the country since an opposition party was disbanded in February.
Protests have been held, but had been swiftly halted by Covid-19 limits. The Thai federal government has denied any involvement in his disappearance.
Three student leaders have been charged over accusations of breaching restrictions in organizing previous protests.
On Sunday, some 50 royalist protesters carrying portraits of the king gathered at the Democracy Monument - the same venue where the anti-government rally will take place later in the day.
Police on Friday also asked a court to revoke the bail for human rights lawyer Anon Nampa, 35 and student activist Panupong Jadnok, 23, whom they arrested on the same charges as Parit last week, Human Rights Watch said.